code: def magnification(self, x, y, kwargs, diff=diff):
          
          f_xx, f_xy, f_yx, f_yy = self.hessian(x, y, kwargs, diff=diff)
          det_A = (1 - f_xx) * (1 - f_yy) - f_xy*f_yx
          return 1/det_A
docs: computes the magnification
          :return: potential