Function bisect 

pub fn bisect(
    f: impl Fn(f64) -> f64,
    a: f64,
    b: f64,
    tol: f64,
    max_iter: usize,
) -> Option<f64>

Bisection method. Requires f(a)*f(b) < 0. Returns the root within tolerance tol, or None if bracket invalid or no convergence.