Creates the model's latent variables

        A latent variable addition requires:

        - Latent variable name - e.g Constant
        - Latent variable prior family - e.g. Normal(0, 1)
        - Variational approximation - e.g. Normal(0, 1)

        Returns
        ----------
        None (adds to self.latent_variables)

def _create_latent_variables(self):
        """ Creates the model's latent variables

        A latent variable addition requires:

        - Latent variable name - e.g Constant
        - Latent variable prior family - e.g. Normal(0, 1)
        - Variational approximation - e.g. Normal(0, 1)

        Returns
        ----------
        None (adds to self.latent_variables)
        """

        self.latent_variables.add_z('Constant', fam.Normal(0,3,transform=None), fam.Normal(0, 3))

        for ar_term in range(self.ar):
            self.latent_variables.add_z('AR(' + str(ar_term+1) + ')', fam.Normal(0,0.5,transform=None), fam.Normal(0, 3))

        for ma_term in range(self.ma):
            self.latent_variables.add_z('MA(' + str(ma_term+1) + ')', fam.Normal(0,0.5,transform=None), fam.Normal(0, 3))