Creates model latent variables

        Returns
        ----------
        None (changes model attributes)

def _create_latent_variables(self):
        """ Creates model latent variables

        Returns
        ----------
        None (changes model attributes)
        """

        self.latent_variables.add_z('Sigma^2 irregular', fam.Flat(transform='exp'), fam.Normal(0,3))

        for parm in range(self.z_no-1):
            self.latent_variables.add_z('Sigma^2 ' + self.X_names[parm], fam.Flat(transform='exp'), fam.Normal(0,3))