Sample KDTree implementation

* Method _standardize should not be part of an Indexer class. It is a generic utility function. Put it in _math.py
* In general, if you can do data processing using numpy element-wise methods rather than loops, use numpy methods - those are always faster.
* Don't implement the whole tree-building logic in CorrelationIndex. Have a general-purpose KDTree data structure do the low-level stuff. Put it in _kdtree.py, for example. Think whether you'll be able to switch that implementation to someone else's if needed.
* Make sure that at least your doctests pass when commiting changes. If you are working on some complex part and you can't make the corresponding test pass, put it into a separate test_something function, not a doctest.