Blender Git Loki

Splitting the BVHTree python type into two distinct subclasses for
DerivedMesh and BMesh support.

This avoids messy code due to mixed data in the same py type. And makes
it clear at any point what kind of data the BVHTree instance represents.

Both types are based on the common BVHTree type, which currently does
not have any functionality of its own. Eventually it could implement
common BVH functions (find-nearest and ray casting) as well as return
read-only info about the size and type of BVH data.

Note: While the BVH functions already use callbacks for providing a
common interface in C, using this for python types would be redundant.
It also still requires specialized data for some details of testing
elements.