Generates an elastic tensor expansion via the fitting function
        defined below in diff_fit

def from_diff_fit(cls, strains, stresses, eq_stress=None,
                      tol=1e-10, order=3):
        """
        Generates an elastic tensor expansion via the fitting function
        defined below in diff_fit
        """
        c_list = diff_fit(strains, stresses, eq_stress, order, tol)
        return cls(c_list)