Returns a generally symmetrized tensor, calculated by taking
        the sum of the tensor and its transpose with respect to all
        possible permutations of indices

def symmetrized(self):
        """
        Returns a generally symmetrized tensor, calculated by taking
        the sum of the tensor and its transpose with respect to all
        possible permutations of indices
        """
        perms = list(itertools.permutations(range(self.rank)))
        return sum([np.transpose(self, ind) for ind in perms]) / len(perms)