Allow filters on quadtree.find

[bmschmidt]

I like to use an optional filter argument with the find method so that the search can return only the nearest relevant point (e.g.: the nearest red point if blue points have been temporarily hidden by a user interaction). The basic fork that works for my purposes is extremely simple; it would take slightly more work to properly handle coincident points.

Something like this is referenced in an existing pull discussion. Happy to file a pull request for just this if desired.

[Fil]

Implemented by #28 ; please test & comment!

demo @ https://observablehq.com/d/07d5853f05318fb2