Add implementations of KDTree / BallTree

[remydubois]

Hello,

thanks for exposing and maintaining this library of Numba examples.

I have a full-Numba implementation of both KDTrees and BallTrees, and wondered whether this would be valuable to add in this repo.
My code is here, and I am willing to enrich it (add a few extra necessary methods) in order to match the minimal functionalities required by most use cases / or match the implementations standards that may reign in this repo.

Let me know,