Function powell 

pub fn powell(
    f: impl Fn(&[f64]) -> f64,
    x0: Vec<f64>,
    tol: f64,
    max_iter: usize,
) -> OptResult

Powell’s method (derivative-free multidimensional minimisation).

Maintains a set of conjugate directions and performs line searches along each. Resets directions periodically to avoid linear dependence.