Inconsistency in image and pixel types using floating point channels.

The typedefs for images and pixels using 32-bit and 64-bit floating point channel values are inconsistent. The 32-bit types defined in *boost/gil/typedefs.hpp* uses `boost::gil::scoped_channel_value` while 64-bit types defined in *boost/gil/io/typedefs.hpp* uses `double`.

`float64_t` is defined in *boost/gil/typedefs.hpp* as `scoped_channel_value` so my guess is that this is the correct channel type for 64-bit images and pixels.