Standardize MPI-parallel and time-dependent data descriptions

[v-dobrev]

The current FMS data structures may be flexible enough to hold the required additional data, however, a specification of what specifically the data looks like is needed.

[vibraphone]

If you have yet to settle on a scheme, it would be helpful to allow for

• mixed time-dependent and static (time-independent) data.
• time-dependent data to be sparse/staggered in time (especially if mesh topology it time-dependent; re-meshing is not typically performed every time step, but will occur at some subset of timesteps).