Gets indices of perturbed elements of the deformation gradient,
        i. e. those that differ from the identity

def get_perturbed_indices(self, tol=1e-8):
        """
        Gets indices of perturbed elements of the deformation gradient,
        i. e. those that differ from the identity
        """
        indices = list(zip(*np.where(abs(self - np.eye(3)) > tol)))
        return indices