add mpich support

[TomMelt]

Add support for mpich

Currently mpich has same problem as intel MPI but it can be fixed by passing an additional option (--pmi-port) to mpirun.

This is related to this closed issue: https://github.com/pmodels/mpich/issues/2063

It would require a few changes

• [ ] add --pmi-port option automatically for mpich MPI.
• [ ] annoyingly mpich --configfile seems to work differently to intel/openMPI. I need to append colons : to the top n-1 lines, because mpich concatenates all lines in the file into one set of args. I think this is a bug but I will check with the mpich team - raised here https://github.com/pmodels/mpich/issues/7290