copy next frame into timestep

def _read_next_timestep(self, ts=None):
    """copy next frame into timestep"""

    if self.ts.frame >= self.n_frames-1:
        raise IOError(errno.EIO, 'trying to go over trajectory limit')
    if ts is None:
        ts = self.ts
    ts.frame += 1

    self.zdock_inst._set_pose_num(ts.frame+1)
    ts._pos = self.zdock_inst.static_mobile_copy_uni.trajectory.ts._pos

    return ts