mpi.h is not included in walberla namespace
mpi.h which is included in MPIWrapper.h is not in walberla namespace.
This leads to problems when build an App that is not inside the walberla namespace because it can't use e.g. walberla:MPI_Recv and will therefore not work with WALBERLA_BUILD_WITH_MPI
just adding namespace walberla
does not work see:
https://i10git.cs.fau.de/thoennes/walberla/commit/f6297e4fdebb68a27b34a72d10d02c991c46c5f5