in ['2-point', '3-point', 'cs']`` the relative step size to
    use for numerical approximation of `jac`. The absolute step
    size is computed as ``h = rel_step * sign(x) * max(1, abs(x))``,
    possibly adjusted to fit into the bounds. For ``method='3-point'``
    the sign of `h` is ignored. If None (default) then step is selected
    automatically.
r