- simplify and rationalize some of the broadcast methods for more code
reuse.
The bottom level UPstream::broadcast is now always to/from "root=0".
This was previously passed as a default parameter, but never used
anything other than '0' in the code. Fixing it as '0' makes it
consistent with the 'top-down' logical for node-based broadcast.
- split/duplicate functionality
- rework inter-node/intra-node handling to allow selection of
splitting based on 'shared' or hostname (default).
- always creates node communicators at startup:
* commInterNode() - between nodes
* commLocalNode() - within a node
- world-comm is now always a duplicate of MPI_COMM_WORLD to provide
better separation from other processes.
NB:
the inter-node comm is a slight exception to other communicators
in that we always retain its list of (global) ranks, even if the
local process is not in that communicator.
This can help when constructing topology-aware patterns.
FIX: non-participating ranks still had knowledge of their potential siblings
- after create by group, the procIDs_ of non-participating ranks
should be empty (except for the inter-node exception)
