More support for binary quadtrees (in particular, set operations)

[dfriend21]

Much of the literature I've come across on quadtrees makes the assumption that the values are binary. This article:

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.299.334&rep=rep1&type=pdf

is a great overview of quadtrees. But it basically assumes binary values.

The author talks about operations like union and intersection. That'd be cool to implement, but it makes much more sense for binary values than it does for continuous values.

There's a lot of functionality in that paper like this that would be nice to include, but a lot of it makes way more sense with binary values, and that hasn't been my focus. So maybe I should try to provide more support for binary-valued quadtrees?