Extend missing implementations for float16 support

Extends missing implementations for float16 support. Namely, an MPI_Datatype specialization for 16 bit (wchar_t) and an equality check. Also, improves the CMake checks for WALBERLA_BUILD_WITH_HALF_PRECISION_SUPPORT depending on the compiler and adds float16 tests to the CI.

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This merge request is related to the HyTeG merge request !671.

More information about necessary compiler features can be found in this HyTeG MR comment.

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TODO:

• Extend MPI_Datatype by a 16 bits type, i.e. WCHAR.
• Add equality check for float16.
• Create a minimal requirement test for float16 that only checks is_floating_point<walberla::float16>::value == true. (Note: must be static_assert since WALBERLA_CHECK does not result in an error.)
• Enable WALBERLA_BUILD_WITH_HALF_PRECISION_SUPPORT in CI. This must be reactivated, once newer compilers are added to the CI.
• Change WALBERLA_BUILD_WITH_HALF_PRECISION_SUPPORT for different compiler checks in CMakeLists.txt to work if the minimal requirement test compiles.