Giving a warning if trying to create a subcontroller with 0 procs.

This shouldn't be an issue of creating a sub MPI controller with
0 processes in it but some MPI implementations don't like it and
crash so I'm not allowing it since I can't see much of a use case
for this anyways.

Change-Id: Ic7f5f490052336b4b6a022a4de55ddc549b04c44
Merge topic 'MPISubGroup_0proc_warning' into master

00146d36 Giving a warning if trying to create a subcontroller with 0 procs.