Allow removing vertices from Delaunay triangulations

[ianmackenzie]

Currently (well, once #58 is merged), Delaunay triangulations can have vertices added but not removed. It should be possible to adapt an algorithm such as https://hal.inria.fr/inria-00167201/document to provide removal functionality if this becomes necessary.

[ianmackenzie]

There's an interesting question of what happens if the same vertex is added twice and then removed once - should it exist in the result or not? May be better to avoid this ambiguity by forbidding duplicates entirely as proposed in #63.