OpenAcademy spatial index

Prototype spatial index data structure. Based on the [Historical R+ Tree](http://www.cs.ust.hk/faculty/dimitris/PAPERS/ssdbm01.pdf) paper.

Right now, the prototype is for a single feature type / layer id tree. We also do not share tree nodes among data from different versions. (Eventually, a container node in the tree should keep data from different versions; queries, splits etc will need to be careful of this.)

This tree is also disconnected from the object database. Places where it should connect are marked in the code.