Creates design matrix holding the lagged variables

        Parameters
        ----------
        Y : np.array
            The dependent variables Y

        Returns
        ----------
        The design matrix Z

def _create_Z(self,Y):
        """ Creates design matrix holding the lagged variables

        Parameters
        ----------
        Y : np.array
            The dependent variables Y

        Returns
        ----------
        The design matrix Z
        """

        Z = np.ones(((self.ylen*self.lags +1),Y[0].shape[0]))
        return self.create_design_matrix(Z, self.data, Y.shape[0], self.lags)