Function steepest_descent 

pub fn steepest_descent(
    cfg: &OptimConfig,
    f: &dyn Fn(&[f64]) -> f64,
    grad: Option<&dyn Fn(&[f64]) -> Vec<f64>>,
    x0: &[f64],
) -> OptimResult

Minimize f from x0 by steepest descent. grad falls back to central finite differences when absent.