spatial: R-tree

Feature request: add an R-tree implementation to spatial.

What are you trying to do?

Given an arbitrary polyhedron represented as an array of polygons (faces), search the array for faces that are within a given region.

What did you try?

Linear searches.

How does Gonum not allow you to achieve your goal?

The closest thing gonum has is spatial/kdtree, which stores points not regions, and cannot be queried by region.

Are you able to help contribute the feature?

I am willing to help, but this is definitely not my area of expertise.