Refine propagation algorithm.

[rkaminsk]

The full propagation algorithm only considers edges between vertices already added to the graph. It should be relatively easy to mark vertices that lead to vertices giving rise to such edges (one can even ignore edges where the potential difference is positive). Only marked vertices have to be considered during propagation. In a first step one could add some code to count how often this situation arises to see if this can help speed up propagation.