Function convex_hull_apply 

pub fn convex_hull_apply(
    n_tax: usize,
    taxa_labels: &[String],
    splits: &[ASplit],
    graph: &mut PhyloSplitsGraph,
) -> Result<()>

Apply the convex hull algorithm to build a split network from splits. n_tax is the number of taxa (1-based semantics). splits is 0-based but we treat ids as 1..=nsplits in bitsets and logs.