Function constrained_func 

pub fn constrained_func<F, G>(
    bounds: Vec<Bound>,
    f: F,
    g: G,
) -> ConstrainedFunc<F, G>
where
    F: Fn(&[f64]) -> f64,
    G: Fn(&[f64]) -> Vec<f64>,

A ConstrainedProblem defined inline by closures.