Cleanup cmake MPI module

Summary

The old cmake file DuneMPI.cmake mixed two independent steps: find_package(MPI) and add_dune_mpi_flags(). This MR renames this file to AddMPIFlags.cmake following the naming scheme used for several other find packages.

Changes:

• Rename DuneMPI.cmake into AddMPIFlags.cmake
• Deprecate the old file
• Move the find_package(MPI) to DuneCommonMacros
• Replace the manual extraction of compile flags and include directories by the imported target MPI::MPI_C
• Do not call the global include_directories for mpi include path.
• Resolve an inconsistency with the MPI flags when used with enable_all_packages or add_dune_mpiflags