abstract away use of MPI communicators (ie make MPI_COMM_WORLD optional)

[jeffhammond]

Using MPI_COMM_WORLD is bad, although it doesn't matter much in GA/ARMCI applications that do not use MPI directly.

We should abstract away the use of communicators in GA/ARCMI so that applications can initialize GA/ARMCI as proper MPI libraries.

I have implemented this in ARMCI-MPI already: pmodels/armci-mpi#26.