Function split_edge 

pub fn split_edge(
    verts: &mut Vec<[f64; 3]>,
    tris: &mut Vec<[usize; 3]>,
    v0: usize,
    v1: usize,
)

Split edge (v0, v1) in the mesh by inserting a midpoint vertex.

All triangles containing that edge are split into two. Updates verts and tris in place.