Add support for inter-communication

[AndrewGaspar]

The code currently operates under the assumption that all communicators are intra-communicators, but inter-communicators have slightly different semantics that should be captured in the type system.

There are places we check the value of MPI_Comm_size for safety purposes, but should actually be MPI_Comm_remote_size when dealing with an inter-communicator. Considering adding a routine fn target_size(&self) -> Rank on the Communicator trait with no default implementation. Evaluates to self.size() on intra-communicators and self.remote_size() on inter-communicators.

The proposed implementation would:

• Capture inter-communicators in the type system as separate entities from intra-communicators
• Add support for creating inter-communicators via MPI_Intercomm_create
• Add support for creating an intra-communicator from an inter-communicator via MPI_Intercomm_merge

I have related changes in the AndrewGaspar/intercomm branch, but I'll probably restructure that branches' changes and submit under a separate branch.