More BVH functionality
Examples:
- node level iterator (pre-order)
- find function (point only? NN? ray? enclosing AABB?)
subtree(node_*_iterator it)
- Find aabb enclosing
- Find aabb inside
- Find aabb up to size
- bvh_view class with node ptr/iter and base bvh ptr?
- bvh_level struct containing start_node ptr and node count?