def ra_indices_for_traj(self, traj):
        """
        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
        """
        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([])
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