code: def _get_coords_by_index(self, coords_array, index):
          
          if hasattr(index, '__iter__'):
              # then we want the center-of-mass coordinates
              # TODO: clean this up
              return np.average([_value(coords_array[i]) for i in index],
                                weights=[self._get_mass_by_index(i) for i in index])
          else:
              return coords_array[index]
docs: where index can either by an integer or a list of integers (returns some of masses)
          coords_array should be a single array (xs, ys, or zs)