Function minimize 

pub fn minimize(
    f: impl Fn(&[f64]) -> (f64, Vec<f64>),
    x0: Vec<f64>,
    options: BfgsOptions,
) -> BfgsResult

Minimize f from x0. f returns the objective and its gradient.

Always returns the best point found; whether that point is good enough is the caller’s question (it knows what the objective means), which is why this reports no “converged” flag of its own.