physics-bh: first working beta

This commit finally introduces the Barnes-Hut algorithm, complete with an octree->generator and an octree->cleaner. The builder works fine most of the time, unless a particle flies off into the distance. Then it blows up and takes over all of your RAM. The cleaner works very well. I could squeeze more performance by tweaking the score each cell gets. And by having a gigantic array of octrees and wiring them using pointers. That'd be a nice way of doing it as we could always reallocate. The cellsum object in the octrees seems a bit buggy so I'll see what's wrong. Anyway, enough banter, I'm sick, I got HL2 to replay and a Barnes-Hut physics algorithm to tweak to my heart's content.

"The fuits of my labour! Smelly, small and few, but sweet nevertheless, for they came from my own garden."