Speed-up through graph caching

[sebhoerl]

We could probably speed up the algorithm quite a bit by caching graph information. We need to think about how to do that. Basically, we would check whether a certain edge is still valid, and then delete the ones that are not valid anymore. On the other hand, it might happen that edges that were previously invalid, become valid now, so actually we would still need to check all possible combinations. But maybe if we do not do this check, then this can be seen as a speed-up heuristic.