code: def gamma(self, x, y, kwargs, diff=diff):
          
          f_xx, f_xy, f_yx, f_yy = self.hessian(x, y, kwargs, diff=diff)
          gamma1 = 1./2 * (f_xx - f_yy)
          gamma2 = f_xy
          return gamma1, gamma2
docs: computes the shear
          :return: gamma1, gamma2