Concept checking fail for indexed_image

### Minimal Working Example (in C++)

```
cd libs/gil
b2 define=BOOST_GIL_USE_CONCEPT_CHECK toolbox/test
```

### Actual behavior

```
gcc.compile.c++ ../../bin.v2/libs/gil/toolbox/test/test.test/gcc-5.5.0/debug/threading-multi/visibility-hidden/indexed_image_test.o
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp: In instantiation of ‘void boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept::constraints() [with Iterator = unsigned char*]’:
../../boost/concept/detail/has_constraints.hpp:32:62: required by substitution of ‘template boost::concepts::detail::yes boost::concepts::detail::has_constraints_(Model*, boost::concepts::detail::wrap_constraints*) [with Model = boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept]’
../../boost/concept/detail/has_constraints.hpp:42:5: required from ‘const bool boost::concepts::not_satisfied >::value’
../../boost/concept/detail/has_constraints.hpp:45:51: required from ‘struct boost::concepts::not_satisfied >’
../../boost/concept/detail/general.hpp:51:8: required from ‘struct boost::concepts::requirement_)>’
../../boost/concept_check.hpp:50:7: required from ‘void boost::function_requires(Model*) [with Model = boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept]’
../../boost/gil/concepts/concept_check.hpp:45:62: [ skipping 13 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
../../boost/concept/detail/general.hpp:51:8: required from ‘struct boost::concepts::requirement_ >)>’
../../boost/gil/locator.hpp:229:5: required from ‘class boost::gil::memory_based_2d_locator >’
../../boost/gil/image_view.hpp:55:48: required from ‘class boost::gil::image_view > >’
../../boost/gil/image.hpp:47:50: required from ‘class boost::gil::image >’
../../boost/gil/extension/toolbox/image_types/indexed_image.hpp:245:54: required from ‘class boost::gil::indexed_image > > >’
toolbox/test/indexed_image_test.cpp:30:62: required from here
../../boost/gil/concepts/pixel_iterator.hpp:94:41: error: ‘memunit_step’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
std::ptrdiff_t bs = memunit_step(it);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:167:23: note: ‘template std::ptrdiff_t boost::gil::memunit_step(const boost::gil::planar_pixel_iterator&)’ declared here, later in the translation unit
inline std::ptrdiff_t memunit_step(const planar_pixel_iterator&) { return sizeof(typename std::iterator_traits::value_type); }
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:97:30: error: ‘memunit_advanced’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
it = memunit_advanced(it, 3);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:188:36: note: ‘template boost::gil::planar_pixel_iterator boost::gil::memunit_advanced(const boost::gil::planar_pixel_iterator&, std::ptrdiff_t)’ declared here, later in the translation unit
inline planar_pixel_iterator memunit_advanced(const planar_pixel_iterator& p, std::ptrdiff_t diff) {
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:98:45: error: ‘memunit_distance’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
std::ptrdiff_t bd = memunit_distance(it, it);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:170:23: note: ‘template std::ptrdiff_t boost::gil::memunit_distance(const boost::gil::planar_pixel_iterator&, const boost::gil::planar_pixel_iterator&)’ declared here, later in the translation unit
inline std::ptrdiff_t memunit_distance(const planar_pixel_iterator& p1, const planar_pixel_iterator& p2) {
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:101:24: error: ‘memunit_advance’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
memunit_advance(it,3);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:183:13: note: ‘template void boost::gil::memunit_advance(boost::gil::planar_pixel_iterator&, std::ptrdiff_t)’ declared here, later in the translation unit
inline void memunit_advance(planar_pixel_iterator& p, std::ptrdiff_t diff) {
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp: In instantiation of ‘void boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept::constraints() [with Iterator = const unsigned char*]’:
../../boost/concept/detail/has_constraints.hpp:32:62: required by substitution of ‘template boost::concepts::detail::yes boost::concepts::detail::has_constraints_(Model*, boost::concepts::detail::wrap_constraints*) [with Model = boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept]’
../../boost/concept/detail/has_constraints.hpp:42:5: required from ‘const bool boost::concepts::not_satisfied >::value’
../../boost/concept/detail/has_constraints.hpp:45:51: required from ‘struct boost::concepts::not_satisfied >’
../../boost/concept/detail/general.hpp:51:8: required from ‘struct boost::concepts::requirement_)>’
../../boost/concept_check.hpp:50:7: required from ‘void boost::function_requires(Model*) [with Model = boost::gil::detail::RandomAccessIteratorIsMemoryBasedConcept]’
../../boost/gil/concepts/concept_check.hpp:45:62: [ skipping 14 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
../../boost/gil/locator.hpp:229:5: required from ‘class boost::gil::memory_based_2d_locator >’
../../boost/gil/image_view.hpp:55:48: required from ‘class boost::gil::image_view > >’
../../boost/gil/image_view.hpp:68:54: required from ‘class boost::gil::image_view > >’
../../boost/gil/image.hpp:47:50: required from ‘class boost::gil::image >’
../../boost/gil/extension/toolbox/image_types/indexed_image.hpp:245:54: required from ‘class boost::gil::indexed_image > > >’
toolbox/test/indexed_image_test.cpp:30:62: required from here
../../boost/gil/concepts/pixel_iterator.hpp:94:41: error: ‘memunit_step’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
std::ptrdiff_t bs = memunit_step(it);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:167:23: note: ‘template std::ptrdiff_t boost::gil::memunit_step(const boost::gil::planar_pixel_iterator&)’ declared here, later in the translation unit
inline std::ptrdiff_t memunit_step(const planar_pixel_iterator&) { return sizeof(typename std::iterator_traits::value_type); }
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:97:30: error: ‘memunit_advanced’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
it = memunit_advanced(it, 3);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:188:36: note: ‘template boost::gil::planar_pixel_iterator boost::gil::memunit_advanced(const boost::gil::planar_pixel_iterator&, std::ptrdiff_t)’ declared here, later in the translation unit
inline planar_pixel_iterator memunit_advanced(const planar_pixel_iterator& p, std::ptrdiff_t diff) {
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:98:45: error: ‘memunit_distance’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
std::ptrdiff_t bd = memunit_distance(it, it);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:170:23: note: ‘template std::ptrdiff_t boost::gil::memunit_distance(const boost::gil::planar_pixel_iterator&, const boost::gil::planar_pixel_iterator&)’ declared here, later in the translation unit
inline std::ptrdiff_t memunit_distance(const planar_pixel_iterator& p1, const planar_pixel_iterator& p2) {
^
In file included from ../../boost/gil/concepts/image_view.hpp:16:0,
from ../../boost/gil/concepts/image.hpp:14,
from ../../boost/gil/concepts.hpp:17,
from ../../boost/gil/color_base_algorithm.hpp:11,
from ../../boost/gil/color_base.hpp:12,
from ../../boost/gil/pixel.hpp:12,
from ../../boost/gil/bit_aligned_pixel_reference.hpp:11,
from ../../boost/gil/bit_aligned_pixel_iterator.hpp:11,
from ../../boost/gil/algorithm.hpp:11,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/concepts/pixel_iterator.hpp:101:24: error: ‘memunit_advance’ was not declared in this scope, and no declarations were found by argument-dependent lookup at the point of instantiation [-fpermissive]
memunit_advance(it,3);
^
In file included from ../../boost/gil/rgb.hpp:12:0,
from ../../boost/gil/typedefs.hpp:16,
from ../../boost/gil/channel_algorithm.hpp:13,
from ../../boost/gil/color_convert.hpp:11,
from ../../boost/gil/image_view_factory.hpp:11,
from ../../boost/gil/algorithm.hpp:15,
from ../../boost/gil.hpp:12,
from toolbox/test/indexed_image_test.cpp:8:
../../boost/gil/planar_pixel_iterator.hpp:183:13: note: ‘template void boost::gil::memunit_advance(boost::gil::planar_pixel_iterator&, std::ptrdiff_t)’ declared here, later in the translation unit
inline void memunit_advance(planar_pixel_iterator& p, std::ptrdiff_t diff) {
^

"g++" -fvisibility-inlines-hidden -fPIC -m64 -pthread -O0 -fno-inline -Wall -g -fvisibility=hidden -std=c++11 -pedantic -fstrict-aliasing -Wcast-align -Wconversion -Wextra -Wfloat-equal -Wshadow -Wsign-promo -Wstrict-aliasing -Wunused-parameter -DBOOST_ALL_NO_LIB=1 -DBOOST_CHRONO_DYN_LINK=1 -DBOOST_GIL_USE_CONCEPT_CHECK -DBOOST_SYSTEM_DYN_LINK=1 -DBOOST_TEST_DYN_LINK=1 -DBOOST_TEST_NO_AUTO_LINK=1 -DBOOST_TIMER_DYN_LINK=1 -DNOMINMAX -D_CRT_NONSTDC_NO_DEPRECATE -D_CRT_SECURE_NO_WARNINGS -I"../.." -I"/mnt/d/boost.wsl" -c -o "../../bin.v2/libs/gil/toolbox/test/test.test/gcc-5.5.0/debug/threading-multi/visibility-hidden/indexed_image_test.o" "toolbox/test/indexed_image_test.cpp"

...failed gcc.compile.c++ ../../bin.v2/libs/gil/toolbox/test/test.test/gcc-5.5.0/debug/threading-multi/visibility-hidden/indexed_image_test.o...
```

### Expected behavior

Successful compilation.

### Environment

All relevant information like:

- Boost version (see `` ):
- Compiler version: GCC 5.4 and clang 6.0
- Build settings: `-DBOOST_GIL_USE_CONCEPT_CHECK=1`

### References

* #174 - general report on `memunit_*` missing or ambiguous; possibly, `indexed_image` is not the only place failing