Collocated sites (or extremely close) need to be handled as they won't be part of the triangulation

[andreesteve]

delaunator may ignore sites extremely close and skip them from the triangulation (https://github.com/mourner/delaunator-rs/issues/18).

This scenario needs to be handled during voronoi construction. Currently it fails with error:

thread 'main' panicked at 'One or more sites is not reacheable in the triangulation mesh. This usually indicate coincident points.', src/lib.rs:163:9