Sunset support for Ubuntu 16.04 after PCL 1.11.1

There's consensus among the maintainers to not support PCL 1.12 on Vanilla 16.04 at least. We might still keep the CI running till the actual EOL date.

Actions to take in preparation of EOL

• Add a 18.04 CI (currently only clang is active, gcc is not)
• Enable -Werror on all 18.04 CI
• Bump minimum supported versions of all development dependencies:
  • cmake (>= 3.10.2) (tracked in #4264)
  • compilers (gcc >= 8.4, clang >= 10.0) (several available, these are highest we can push to)
  • Eigen (>= 3.3.4)
  • FLANN (>= 1.9.1)
  • Boost (>= 1.65)
  • VTK (>= 7.1.1 , < 9, patch in progress)
  • others
• Remove fixes and hacks for old versions (need to be enumerated)

List of new features available for use/abuse

CMake

• Native support for CUDA language
• googletest support
• OpenMP + Clang support
• FindBoost improvements
• Updates to if command for comparisons
• compile_features instead of flags (eg: C++ 17 polyfill features) (PCL might still remain at C++14)
• Doxygen support
• Clang tidy integration
• cppcheck integration
• and more (<-- That's just 1 release)

Compilers

GCC

• -Wreturn-type
• Stricter template checking
• alignof changes
• null value check

Clang

• new diagnostics
• OpenMP 5 support (Don't know the status in GCC)
• new sanitizers

Eigen

• Fixes for NVCC 9
• looks like bug fixes

Boost

Minor improvements

FLANN

Couldn't find release notes, but here you go

VTK

Not familiar with VTK usage in PCL. Please distil the following:

• VTK 7
• VTK 7.1

List of deprecated features

CMake

• New policies for AUTOMOC and AUTOUIC (Might help with Qt)

Compilers, Eigen, VTK, Boost

(should not affect PCL)

Start with the existing Ubuntu 18.04 CI configuration and the CMake work tracked in #4264. Inventory the unchecked compiler and dependency version changes, then review where old-version fixes and hacks remain. Done means the listed CI and minimum-version actions are addressed and the remaining compatibility work is explicitly enumerated.