Function powell 

fn powell(
    cost: &mut CostFn<'_>,
    start: [f64; 6],
    scale: [f64; 6],
    max_cycles: usize,
    tol: f64,
) -> [f64; 6]

Powell’s direction-set method.

Minimises along each of six directions in turn, then — this is the part that distinguishes Powell from plain coordinate descent — folds the net displacement of the cycle in as a new direction, so a valley running diagonally through parameter space gets followed instead of zigzagged along. The initial directions are the coordinate axes scaled by scale, which is how rotations (radians) and translations (mm) are put on comparable footing.