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