ENH: Added broadcast implementation for RMI triggers

This commit adds a different implementation of TriggerRMIOnAllChildren() and
ProcessRMIs(), which uses collective operations, i.e., MPI broadcast, to send
the messages from the root node (rank 0) to all the satellite ranks. The
underlying implementation that is used internally is controlled by a
member variable, "BroadcastTriggerRMI" that can be set by the user.

Change-Id: Ifa850372d0f36c75522897632574eb68a427436d
Merge topic 'use-mpi-collectives-in-rmi-triggers' into master

1a80cf61 ENH: Added broadcast implementation for RMI triggers