def D_dt(self, z_lens, z_source):
        """
        time-delay distance

        :param z_lens: redshift of lens
        :param z_source: redshift of source
        :return: time-delay distance in units of Mpc
        """
        return self.D_xy(0, z_lens) * self.D_xy(0, z_source) / self.D_xy(z_lens, z_source) * (1 + z_lens)
time-delay distance

        :param z_lens: redshift of lens
        :param z_source: redshift of source
        :return: time-delay distance in units of Mpc