fastjet/internal/delaunay: implement voronoi

Created by: Bastiantheone

This PR adds several functions to compute the Voronoi Diagram.

To handle the issue #111 I initially started implementing it where Voronoi points on the outside connect to its neighbor. But then after thinking about it more, I changed it to connect to the Circumcenter of the adjacent root triangle, which is just a dummy triangle. Those points are moved to the first and last position in the slice so that they are easy to identify. They might be useful to get a direction of where the line goes towards.