Undocumented (unused?) call argument in find_cube_differences

The signature for find_cube_differences looks like this

def find_cube_differences(cube_list, oper='VGC', return_type='logging',
                          cube_labels=None, var_name=None)

but the last argument, var_name is not documented in the docstring. It is only used when calling the print_dataframe, and then only in an error message.