Function hypervolume 

pub fn hypervolume(pareto_front: &[Solution], reference_point: &[f64]) -> f64

Calculate the hypervolume indicator.

Uses an exact algorithm for up to 7 objectives:

• 2D: O(N log N) sweep-line staircase
• 3–7D: WFG (Hypervolume by Slicing Objectives) exact algorithm

For 8 or more objectives, where WFG’s O(N^(d-2)) cost is comparable to Monte Carlo, falls back to Monte Carlo sampling with 10,000 samples.