Code to find bounding boxes is very slow

[lukehutch]

While running build_octree, during the initial Determining bounding box stage I get a processing speed on a fast laptop CPU and SSD of 75,083.64/s, which for a .ply file consisting of 19.1B points, takes 7 hours to process. This is not even remotely saturating the SSD's IO bus or the CPU's processing capabilities, so something is very inefficient in the bounding box calculation code.