Feature request: N to M restart capability

The *MIRGE-Com* simulation application would like an N-to-M restart capability (i.e. where simulations originally run on N processors may be restarted on M processors) so that the simulations can adapt to changing resource availability.

This is an important capability for running production-scale problems on lab-based machines where resource availability is highly variable, and often includes dedicated pushes where we have temporary access to large portions of the machine.

The capability need not be fully integrated with the simulation code; it is OK if we need to run an "adapter" code in-between runs to serialize and then re-partition N-to-M.