Remove config.h includes related to MPI

Simon Praetoriusrequested to merge
feature/mpi-include into master

Summary

MPI Flags are set by the cmake macro add_dune_mpi_flags, like HAVE_MPI for example. It is not necessary to include the config.h file for this. This MR cleans up all the source files that previously included the config.h. It also extends the add_dune_mpi_flags macro by additional utility flags to prevent the inclusion of the mpi c++-bindings.

Full System Test

Edited by Simon Praetorius

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