code: def check_solver(self, kwargs_lens, kwargs_ps, kwargs_cosmo={}):
          
          if self._solver is True:
              image_x, image_y = kwargs_ps[0]['ra_image'], kwargs_ps[0]['dec_image']
              image_x, image_y = self.real_image_positions(image_x, image_y, kwargs_cosmo)
              dist = self._solver_module.check_solver(image_x, image_y, kwargs_lens)
              return np.max(dist)
          else:
              return 0
docs: test whether the image positions map back to the same source position
          :param kwargs_lens:
          :param kwargs_ps:
          :return: Euclidean distance between the rayshooting of the image positions