Enable save_glo_info_and_grids/save_velocities in old diagnostics

• Move save_restart_dist_fn to old diagnostics and generalise

  Allows both modules to call it, and enables adding save_glo_info_and_grids and save_velocities to old diagnostics

• Enable save_glo_info_and_grids, save_velocities in old diagnostics

Now we have a shared config between the two diagnostics modules, this basically required no new code, just rearrangement of existing code, and the addition of module-level variables (though we could just use the module-level config instead!)