dict {group name -> group indices}

def indices(self):
        """ dict {group name -> group indices} """
        if len(self.groupings) == 1:
            return self.groupings[0].indices
        else:
            label_list = [ping.labels for ping in self.groupings]
            keys = [com.values_from_object(ping.group_index)
                    for ping in self.groupings]
            return get_indexer_dict(label_list, keys)