code: def get_polarization_change_norm(self, convert_to_muC_per_cm2=True, all_in_polar=True):
          
          polar = self.structures[-1]
          a, b, c = polar.lattice.matrix
          a, b, c = a / np.linalg.norm(a), b / np.linalg.norm(
              b), c / np.linalg.norm(c)
          P = self.get_polarization_change(convert_to_muC_per_cm2=convert_to_muC_per_cm2,
                                           all_in_polar=all_in_polar).ravel()
          P_norm = np.linalg.norm(a * P[0] + b * P[1] + c * P[2])
          return P_norm
docs: Get magnitude of difference between nonpolar and polar same branch
          polarization.