Make a persistent physical node communicator

Make a persistent physical node communicator

Rather than make and re-make this communicator multiple times for for
almost all common run configurations, build one, and keep it in a
place distinct from commrec (which has too many responsibilities
already).

Note that the master thread-MPI rank builds a fake one initially,
because MPI is not yet initialized, then coordinates to build the real
one with the newly launched threads. However that communicator is not
actually used until after the execution paths have re-joined.

Simplifies IThreadAffinityAccess while retaining the ability to test
the affinity-setting logic without requiring specific attributes of
the node. Note that some of the tests already required that a
particular MPI environment exists.

Change-Id: Ie0800b07a65f9579ddc8af8f418cd14b85671de7