code: def _psi(self, x, y, q, s):
          
          return np.sqrt(q**2 * (s**2 + x**2) + y**2)
docs: expression after equation (8) in Keeton&Kochanek 1998
  
          :param x:
          :param y:
          :param q:
          :param s:
          :return: