Handle Multiple Communicators in MPI-Sim layer

We should add the support for multiple communicators in the MPI Simulation layer (Right now we assume MPI_COMM_WORLD for all cases). One of the traces on the Design Forward website, MiniDFT is using MPI_Comm_Split and MPI_Comm_Dup calls for communication. For now, we could issue a warning and exit for application traces that use multiple communicators to prevent incorrect performance prediction.