Function assign_angles_to_edges 

pub fn assign_angles_to_edges(
    ntaxa: usize,
    splits: &SplitsBlock,
    cycle: &[usize],
    g: &mut PhyloSplitsGraph,
    forbidden: Option<&FixedBitSet>,
    total_angle: f64,
)

Assign angles to edges: compute split directions (per split id) on a circle, then stamp on edges unless forbidden.