Updated code checks with distance restraints

Ensemble-averaged distance restraints require -multi, and it is
also reasonable to do REMD with distance restraints, which also
requires -multi. So checks for the ensemble-averaging case need
to be more sensitive to their context.

In fact, ensemble-averaging is probably functional only with PD and
one processor per system, since that was probably all that was
available when it was built.

Fixes #613, refs #1117

Change-Id: Ia6f1bb4eb82eab3c7c249638cd3a5a5d1f707132