Separated core functionality tests from index structure tests
Added generic logic for testing index structures, added implementation of point kd-tree from Bentley's 1975 paper and remove util.hpp, since the only functions in there have been moved to types.hpp
Added Multigrid class, which implements the Multigrid Tree index structure
Added generic hash-based index structure and a subclass of this which hashes points based on their bit values
Added implementation of the Pyramid Tree structure described in Berchtold et al.'s 1998 paper