Teuchos: Add aggregate versions of CommRequest & CommStatus
Created by: mhoemmen
@trilinos/teuchos @trilinos/tpetra Epic: #767.
Add versions of CommRequest and CommStatus that store an array of MPI_Request resp. MPI_Status. This will avoid copying in/out for MPI_Waitall calls, and allow experimentation with communication / computation overlap (e.g., by calling MPI_Waitsome).
This is relevant to @trilinos/tpetra communication performance.