Interface for tree traversal / walking of BallTree/KDTree

[dgleich]

It'd be great to have an interface to walk the tree structures. This could be used internally by some of the search options and externally as in a few cases I have below.

see https://github.com/dgleich/GraphPlayground.jl/blob/main/src/manybodyforce.jl for usage (which just broke given one of your changes :) ... which is what I should expect for using undocumented internals!) and https://discourse.julialang.org/t/packages-for-quadtrees-in-julia/113078/3

So I'm trying to get a more useful walk interface...

I have a few ideas, but I'm trying to solicit use cases at the moment.

• my use cases above
• inrange_kernel in NearestNeighbors.

Are there others? The plan is to submit a pull request at some point with an initial take on how to make this generally useful.