Switch to c++17

Summary

Upgrade to the C++ 2017 standard

Rationale

OTB is currently compiled with c++14. Now that the c++17 standard is well implemented by the compilers, we could upgrad to c++17. This would come with a lot of advantages:

• There are a lot of TODOs in the code telling that when we switch to c++17 we will be able to simplify that part of code.
• The new std::clamp is useful (and could simplify !797 (merged)).
• We will be able to remove the homemade emulation of the string_view, and use the standard one.
• A bunch of new algorithms and mathematical functions.
• Some new features concerning the templates
• The new if constexpr will be handful to simplify the code.
• Many other goodies.

This update means we drop support for older compilers (like VisualC++14).

Implementation Details

Modify CMAKE_CXX_STANDARD to 17 in main CMakeFiles.txt.

Remove the REGISTER keyword from 6S code.

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 👍 votes from core developers, no 👎 vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit