Returns a "voigt"-symmetrized tensor, i. e. a voigt-notation
        tensor such that it is invariant wrt permutation of indices

def voigt_symmetrized(self):
        """
        Returns a "voigt"-symmetrized tensor, i. e. a voigt-notation
        tensor such that it is invariant wrt permutation of indices
        """
        if not (self.rank % 2 == 0 and self.rank >= 2):
            raise ValueError("V-symmetrization requires rank even and >= 2")

        v = self.voigt
        perms = list(itertools.permutations(range(len(v.shape))))
        new_v = sum([np.transpose(v, ind) for ind in perms]) / len(perms)
        return self.__class__.from_voigt(new_v)