add support for MPI groups

[jeffhammond]

GAMESS needs this.

[JorgeG94]

I see that there's already some work done in https://github.com/jeffhammond/vapaa/blob/main/source/mpi_group_f.F90 do you have a vague plan for this? I can help with this.

[jeffhammond]

I can probably pound it out in a day with Cursor the way I did the RMA stuff. Writing tests is probably the most useful thing.

[JorgeG94]

I can write tests! :D