code: def _split_indices(self, concat_inds):
          
          clengths = np.append([0], np.cumsum(self.__lengths))
          mapping = np.zeros((clengths[-1], 2), dtype=int)
          for traj_i, (start, end) in enumerate(zip(clengths[:-1], clengths[1:])):
              mapping[start:end, 0] = traj_i
              mapping[start:end, 1] = np.arange(end - start)
          return mapping[concat_inds]
docs: Take indices in 'concatenated space' and return as pairs
          of (traj_i, frame_i)