Fix MaterialStrain.plot()

[adtzlr]

for materials with state variables. The state variables are now correctly handled during the Newton iterations for the transversal stretches.

Fixes #752

def fun(λ3):
    λ2 = λ3
    F = eye * np.array([λ1, λ2, λ3]).reshape(1, 3, 1, -1)
    if self.statevars_included:
        statevars = self.statevars
        if statevars is None:
            statevars = np.zeros((*self.umat.x[-1].shape, 1, 1))
        P = np.zeros_like(F)
        for increment, defgrad in enumerate(F.T):
            P[..., [increment]], statevars = self.umat.gradient(
                [F[..., [increment]], statevars]
            )
    else:
        P, statevars = self.umat.gradient([F, None])
    return P[2, 2].ravel()