Function halley 

pub fn halley(
    cfg: &Solver1d,
    f: impl Fn(f64) -> f64,
    df: impl Fn(f64) -> f64,
    d2f: impl Fn(f64) -> f64,
    x0: f64,
) -> Root

Halley iteration x - 2 f f' / (2 f'^2 - f f'').