Commit for comments for the KNN search.

This represents the core of the KNN search, but there are still some outstanding items:

~~1) More unit tests are needed, starting with tests of KNNQuery itself. This is the top priority.~~
~~2) Replacement of SomeGeoHashes: a "touching" GeoHash generator needs to replace the BoundingBoxGeoHashIterator.~~
   Also, distances need to be ~~cached and~~ switched over to geodetic calculations.
~~3) The NearestNeighbor SimpleFeature PriorityQueue needs to be bounded to hold "k" elements only.~~
4) Distance calculations for SimpleFeatures should be done via transforms instead of client-side.
5) A KNN iterator which does the NearestNeighbor sorting and bounding  server-side could then be introduced.
Actually, the type annotation is tricky. Instead I'm going to wrap (sf,dist) into a case class. This improves the readability significantly.