'(bool) Whether or not currently active representation matrices are
time-invariant'

@property
def time_invariant(self):

    if (self._time_invariant is None):
        return (self._design.shape[2] == self._obs_intercept.shape[1] == self._obs_cov.shape[2] == self._transition.shape[2] == self._state_intercept.shape[1] == self._selection.shape[2] == self._state_cov.shape[2])
    else:
        return self._time_invariant
