Calculates the negative log-likelihood of the model for Normal family

        Parameters
        ----------
        beta : np.ndarray
            Contains untransformed starting values for latent variables

        Returns
        ----------
        The negative logliklihood of the model

def normal_neg_loglik(self, beta):
        """ Calculates the negative log-likelihood of the model for Normal family

        Parameters
        ----------
        beta : np.ndarray
            Contains untransformed starting values for latent variables

        Returns
        ----------
        The negative logliklihood of the model
        """     
        mu, Y = self._model(beta)
        return -np.sum(ss.norm.logpdf(Y, loc=mu, scale=self.latent_variables.z_list[-1].prior.transform(beta[-1])))