Fix Graph Physics to Stop Explosion

[avose]

The physics calculation still has issues with large directories, like the ".git" dir; the whole thing doesn't just run slow, it blows up.. Maybe capping this is enough, or perhaps also starting the initial state such that the nodes' random positions are spread out more to reduce the forces involved.

[avose]

OK, this didn't turn out to be too hard. I put a cap on the maximum force applied to the node positions, and spread them out a bit more on initial graph creation. Even very large directories seem stable (if a bit slow).