KD trees (used by Lucene's new dimensional points) excel at finding "nearest neighbors" to a given query point ... I think we should add this to Lucene's sandbox as:
I only implemented the 1 nearest neighbor for starters ... I think we can easily generalize this in the future to K nearest.
It could also be generalized to more than 2 dimensions, but for now I'm making the class package private in sandbox for just the geo2d (lat/lon) use case.
I don't think this should go into 6.0.0, but should go into 6.1: it's a new feature, and we need to wrap up and ship 6.0.0 already