Adds leverage term to the model

        Returns
        ----------
        None (changes instance attributes)

def add_leverage(self):
        """ Adds leverage term to the model

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

        if self.leverage is True:
            pass
        else:
            self.leverage = True
            self.z_no += 1
            
            for i in range(len(self.X_names)*2+3):
                self.latent_variables.z_list.pop()            

            for parm in range(len(self.X_names)):
                self.latent_variables.add_z('Vol Beta ' + self.X_names[parm], fam.Normal(0,10,transform=None), fam.Normal(0,3))

            for parm in range(len(self.X_names)):
                self.latent_variables.add_z('Returns Beta ' + self.X_names[parm], fam.Normal(0,10,transform=None), fam.Normal(0,3))

            self.latent_variables.add_z('Leverage Term', fam.Flat(transform=None), fam.Normal(0,3))
            self.latent_variables.add_z('v', fam.Flat(transform='exp'), fam.Normal(0,3))
            self.latent_variables.add_z('Returns Constant', fam.Normal(0,3,transform=None), fam.Normal(0,3))
            self.latent_variables.add_z('GARCH-M', fam.Normal(0,3,transform=None), fam.Normal(0,3))
            self.latent_variables.z_list[-3].start = 2.0