Remove cxx feature checks that are already in c++17 standard library

Simon Praetoriusrequested to merge
feature/reduce-cxx-feature-checks into master

Summary

This MR removes some c++ feature checks from cmake, since the features are already standard:

  • DUNE_HAVE_CXX_BOOL_CONSTANT
  • DUNE_HAVE_CXX_EXPERIMENTAL_BOOL_CONSTANT
  • DUNE_HAVE_HEADER_EXPERIMENTAL_TYPE_TRAITS
  • DUNE_HAVE_CXX_APPLY
  • DUNE_HAVE_CXX_EXPERIMENTAL_APPLY
  • HAVE_IS_INDEXABLE_SUPPORT

The fallback implementations are removed and the inclusion into the Dune::Std namespace is marked deprecated. This does typically not work for using declarations. Thus, the functions are marked deprecated in Doxygen only.

Edited by Simon Praetorius

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