code: def g_reuss(self):
          
          return 15. / (8. * self.compliance_tensor.voigt[:3, :3].trace() -
                        4. * np.triu(self.compliance_tensor.voigt[:3, :3]).sum() +
                        3. * self.compliance_tensor.voigt[3:, 3:].trace())
docs: returns the G_r shear modulus