Fix wrapper.
Fix wrapper.

For "reasons", boost assumes that the MPI C++ wrapper is called mpic++.

This is not true for /many/ MPI implementations including Intel MPI.

I've forced it to use mpicxx which is a correct wrapper for both the Intel
MPI and OpenMPI.

UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts,UCL-RITS/rcps-buildscripts