Split out SpatialTreeInterface?

[asinghvi17]

Now that I've used it in a couple of places it's feeling pretty mature - perhaps enough so to move it to a separate package.

The question then becomes whether it should live in a lib/ here or whether it needs to be a separate repo? I'd be content with it living in a lib/ but curious what other folks think.

cc @meggart @evetion @rafaqz

[evetion]

I like to try it in GeoDataFrames, but we depend already on GO, so no need yet for splitting it out? Might be good to test drive it there for Geometry vector?