[bboxtree] Add method to intersection two AABB trees
We can collide two AABB trees efficiently now. Thus, AABB trees of two grids can be intersected. This returns a pair of vectors. They have the same length. Each entry contains the element index of the one grid in the first vector and the intersecting element of the second grid in the second vector. This implements primitive tests for intersection of two AABB trees only for 3d with 1d grids for now. If you want to intersect grids of other dimensions implement the method collides in the geometry collision class added with this commit.