teach TransientGridSearch to re-use atoms and Fmn from disk
GridSearch
already has logic to re-use results from disk, if inputs match. TransientGridSearch
inherits this but isn't able to do the same for the actual transient-relevant outputs, just the standard persistent-2F grid file.
Best way to do this would probably be through overriding the check_old_data_is_okay_to_use()
method.