physics, all: rewrite, split, add barnes-hut algorithm(W.I.P.)

Octree generation works perfectly right now. Still need to work on the actual force calculations. Splitting was done quickly so there's still some cruft left over, which I'll clean when I get around to it. The actual algorithm choosing code was inspired by the mplayer scaling algorithm kernel chooser. And it works damn fine, let me tell you that. Alright, back to work.