docs: def clearFixedEffect(self):
          """ erase all fixed effects """
          self._A = []
          self._F = []
          self._B = []
          self._A_identity = []
          self._REML_term = []
          self._n_terms = 0
          self._n_fixed_effs = 0
          self._n_fixed_effs_REML = 0
          self.indicator = {'term':np.array([]),
                              'row':np.array([]),
                              'col':np.array([])}
          self.clear_cache('Fstar','Astar','Xstar','Xhat',
                           'Areml','Areml_eigh','Areml_chol','Areml_inv','beta_hat','B_hat',
                           'LRLdiag_Xhat_tens','Areml_grad',
                           'beta_grad','Xstar_beta_grad','Zstar','DLZ')