physics-bh: scoring system for cleanup, statistics, prevent overflows

The scoring system is quite nice on paper. Decrease all leaf nodes' scores and free 'em once they go past a certain spot. Don't decrease the score of any non-leaf cells. Some problems though, we get into callocating frenzy and take every bit of memory sometimes. It's even worse since we don't use malloc and calloc actually initializes the memory blocks AND the kernel is not lazy and gives them straight away. With malloc you could probably not even see this problem. I suppose the problem comes from incorrectly setting the halfdist of the root cell. Will solve this problem today. I've got an idea on how to run the whole octree generation in multiple threads now too. But first, have to find exactly where the problem is and do the usual: create an exception, see it fail, find out why it fails and make a general process.