Draft: Add MPI flags if ParMETIS is used

The MPI flags set by FindParMETIS and add_dune_mpi_flags are not aligned. This MR uses add_dune_mpi_flags to set the correct flags.

How to do this properly? ParMETIS depends on METIS and MPI. Where to set this dependency? We have a library dependency that can be enforced by LINK_LIBRARIES parameter in the FindParMETIS module, but where to set the dune related flags? Where to set the special MPI flags we are using? I have now added the add_dune_mpi_flags call inside the add_dune_parmetis_flags macro, but this would link against MPI::MPI_C twice. Probably this is no problem. Is there a better idea?