get_restart_names returns names of variables not required for a restart

[oliverwm1]

fv3gfs.get_restart_names returns a list of all proprerties in dynamics_properties.json and physics_properties.json instead of a list of names of variables required for a clean restart.

Could be fixed by either adding more variables to RESTART_EXCLUDE_NAMES or (better) adding a key to each physics/dynamics property about whether it is required for a restart.

[mcgibbon]

I vote for a required_for_restart key which defaults to False if not present.