Feature request: Find all pairs of points within dist of self in other

[ChHecker]

Hi there! Thanks for this great repo.

I have a feature request, inspired by the k-d tree in scipy:
Could there be a function that finds all points in a KdTree other within a dist for all points in a KdTree self? So basically the scipy function query_ball_tree.

Thanks.

[clbarnes]

We also have use for a tree-to-tree lookup (comparing point cloud similarities for neuroscience), although for us it would be a single nearest neighbor query.

A related (probably simpler) problem is finding, for each point in the point cloud, its nearest neighbors in the same cloud - the current strategy requires traversing the tree from the top for each point which could probably be improved on.