code: def ra_indices_for_traj(self, traj):
          
          assert not self.uniform_stride, "requested random access indices, but is in uniform stride mode"
          if traj in self.traj_keys:
              return self.ra_indices_for_traj_dict[traj]
          else:
              return np.array([])
docs: Gives the indices for a trajectory file index (without changing the order within the trajectory itself).
          :param traj: a trajectory file index
          :return: a Nx1 - np.array of the indices corresponding to the trajectory index