Function assign_angles_to_splits 

pub fn assign_angles_to_splits(
    ntaxa: usize,
    splits: &SplitsBlock,
    cycle: &[usize],
    total_angle: f64,
) -> Vec<f64>

Compute split direction for each split id (1-based) by “mid-angle” between the first/last taxa (in cycle order) that lie on the part not containing cycle[1].