Function second_derivatives 

fn second_derivatives<R: Scalar>(
    scheme: Interpolation,
    taus: &[f64],
    ys: &[R],
) -> Vec<R>

The second derivatives at every knot, solved from the continuity system.

Returns one value per knot — anchor first — so evaluation reads z[i], z[i + 1] for the segment it located and nothing else.