Weighted triangulations and Voronoi power diagrams

[DanielVandH]

I did get started on weighted triangulations, but couldn't get through to fixing it. There are some various comments in the tests about it (see runtests.jl), and a lot of the functionality actually already exists in the package it just doesn't work (the main issue is that submerged vertices aren't being properly detected when they are inserted outside of the triangulation...). The ideas behind weighted triangulations are discussed more in the book by Cheng et al.

Once weighted triangulations exist we can get started on power diagrams and then possibly #87.