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