Add spatial_join

[deanm0000]

https://github.com/msalib/spatial-join

Not sure if you'd prefer to use r star directly but some form of spatial joins would be nice.

[kylebarron]

Spatial joins are already half-implemented in #580. https://github.com/msalib/spatial-join is a useful reference but we can't use it directly because it uses geo objects. Additionally, the spatial joins here will use https://github.com/kylebarron/geo-index, which is more memory-efficient and will be quite a bit faster than rstar for our static data use case.

[deanm0000]

Oh that's awesome. Is this going to be just nearest (or similar) or is the other half (yet to be implemented) going to be other predicates (contains, intersects, etc)?

[kylebarron]

Initially intersects but should be all predicates eventually