add a quadtree to find neighbouring atoms faster

[dickloraine]

Hi, very nice project! I implemented a quadtree to make use of the effect radius being limited. This way finding nearby atoms is much faster, as long as the radii aren't huge. This quadtree only queries rectangles, so the distance check in applyRules is still neccessary, but this should still be faster than checking for circular intersections.

One could add gui-controlls for this, but this is only for performance and has nothing to do with the core functionality. But there is one case, where this would be slower than the current way. If the radii are large, so that nearly all atoms have to be checked anyways, this just adds overhead.

For large numbers of atoms with the default radius, this is a 3-5 times improvement in fps.

[ker2x]

Oh that's amazing i never took the time to do it !