Add support for C++20 concepts
- Some compilers already have support for concepts, hence, it would be a good idea to provide some basic compiler checks to enable it through DUNE.
- Additionally, since some compilers do not provide the concepts library (e.g. clang), some concepts are provided as a fallback implementation.
In particular, this would enable one of my two Grid concept implementations for those using C++20.
I tested this with clang-11. Pipelines for supported compilers seem to be passing.
Usage: dune-grid!377