Function hypervolume 

pub fn hypervolume(
    solutions: &[MultiObjectiveSolution],
    reference_point: &[f64],
) -> f64

Calculate the hypervolume indicator for a set of solutions.

For 2D problems, uses an exact O(N log N) sweep-line algorithm. For 3D+ problems, uses the WFG (Walking Fish Group) exact algorithm.

All solutions must have objective values strictly dominated by reference_point to contribute to the hypervolume.