code: def add_distances(self, indices, periodic=True, indices2=None):
          r
          from .distances import DistanceFeature
          atom_pairs = _parse_pairwise_input(
              indices, indices2, self.logger, fname='add_distances()')
          atom_pairs = self._check_indices(atom_pairs)
          f = DistanceFeature(self.topology, atom_pairs, periodic=periodic)
          self.__add_feature(f)