code: def D_dt(self, H_0, Om0, Ode0=None):
          
          lensCosmo = self._get_cosom(H_0, Om0, Ode0)
          return lensCosmo.D_dt
docs: time delay distance
          :param H_0: Hubble parameter [km/s/Mpc]
          :param Om0: normalized matter density at present time
          :return: float [Mpc]