returns the Voigt-notation compliance tensor,
        which is the matrix inverse of the
        Voigt-notation elastic tensor

def compliance_tensor(self):
        """
        returns the Voigt-notation compliance tensor,
        which is the matrix inverse of the
        Voigt-notation elastic tensor
        """
        s_voigt = np.linalg.inv(self.voigt)
        return ComplianceTensor.from_voigt(s_voigt)