code: def time_delays(self, kwargs_lens, kwargs_ps, kappa_ext=0):
          
          fermat_pot = self.lens_analysis.fermat_potential(kwargs_lens, kwargs_ps)
          time_delay = self.lensCosmo.time_delay_units(fermat_pot, kappa_ext)
          return time_delay
docs: predicts the time delays of the image positions
  
          :param kwargs_lens: lens model parameters
          :param kwargs_ps: point source parameters
          :param kappa_ext: external convergence (optional)
          :return: time delays at image positions for the fixed cosmology