Function spread 

pub fn spread(pareto_front: &[Vec<f64>]) -> f64

Spread / diversity indicator (Δ, Delta).

Measures the extent and uniformity of the Pareto front approximation. Based on Deb et al. (2002): computes the average nearest-neighbour distance for internal points and compares against extreme-point distances.

Returns a value ≥ 0; smaller means better distributed front. Returns 0 for fewer than 2 points.

Notes

This implementation uses the modified spread indicator where the value is computed as the standard deviation of nearest-neighbour distances, normalised by the mean distance.