new virtual Shape::intersect_bbox
implementation of triangle-AABB intersection
octree building updated and simplified with help of this new method
octree made default for Python, it's currently much faster than kd-tree (both building and traversal)
new space partitioning structure: octree
realtime_bunny updated to use octree
plus other files updated to be container type independent (only user programs are supposed to include and use special containers)