Const-qualify various int * arguments in MpiWrapper.h to comply with MPI standard

A number of signatures in core/mpi/MPIWrapper.h were not compliant with the MPI standard (see here and here) in the sense that they had int * arguments that were not const-qualified, contrary to the API defined in the standard. This turned up when we const-qualified some APIs of MpiManager in !707 (merged) .

requested review from @ed94aqyc

assigned to @da15siwa

requested review from @on74yces and removed review request for @ed94aqyc

approved this merge request

merged

mentioned in commit 65fcc780