Python implementation of cover trees, near-drop-in replacement for scipy.spatial.kdtree

This is a Python implementation of cover trees, a data structure for finding
nearest neighbors in a general metric space (e.g., a 3D box with periodic
boundary conditions).

Cover trees are described in two papers hosted here:

http://hunch.net/~jl/projects/cover_tree/cover_tree.html

The implementation here owes a great deal to PyCoverTree, by Thomas Kollar,
Nil Geisweiller, Emanuele Olivetti, which can be found here:

http://github.com/emanuele/PyCoverTree

The API follows that of Anne M. Archibald's KD-tree implementation for scipy
(scipy.spatial.kdtree).  Other than specifying a distance function in the
constructor, this module can be used as a drop-in replacement for kdtree.