physics-bh: Use C11 atomics for shared vars, free octrees upon exit

Yes, it's decimate, not desecrate, no typo here. I have an idea how to thread the octree generation, might be clumsy but I'll give it a go. Also, using C11 atomics instead of a mutex because we already require it(as --std=gnu11). Atomics definately seem the most interesting feature of C11. Pthreads already does everything you need threads to do(except sending signals, that's still clumsy). Also, lua_tonumber always returns double. And the "by specifications" part means I have let to lay my eyes upon a coordinate taking more than 10 decimal digits in any kind of a format, so might as well stick to floats. Also, MiB > MB. Literally and figuratively speaking. Over and out!