Function bfgs 

pub fn bfgs<F, G>(
    f: F,
    grad: G,
    start: Array1<f64>,
    max_iter: usize,
    tol: f64,
) -> Array1<f64>
where
    F: Fn(&Array1<f64>) -> f64,
    G: Fn(&Array1<f64>) -> Array1<f64>,

BFGS optimization with provided gradient.