[BL/MPI.h] In / out buffers must not be aliased

See https://www.mpi-forum.org/docs/mpi-4.1/mpi41-report.pdf page 10

The following error message on JUWELS is fixed with this commit

vtkMPICommunicator.cxx:60    WARN| MPI had an error
------------------------------------------------
Invalid buffer pointer, error stack:
internal_Allgather(133): MPI_Allgather(sendbuf=0x1233940, sendcount=1, MPI_UNSIGNED_LONG, recvbuf=0x1233840, recvcount=1, MPI_UNSIGNED_LONG, MPI_COMM_WORLD) failed
internal_Allgather(85).: Buffers must not be aliased