Function constrained_weight 

pub fn constrained_weight(
    l1: &f32,
    l2: &f32,
    max_delta_step: &f32,
    gradient_sum: f32,
    hessian_sum: f32,
    lower_bound: f32,
    upper_bound: f32,
    constraint: Option<&Constraint>,
) -> f32

Calculate the constraint weight given bounds and a constraint.