fixes skim creation of simplified graph

[janzill]

fixes #247

[janzill]

@pedrocamargo it looks like you add links with a_node==b_node after network simplification and set the compact__id to compact_num_links+1, so the groupby object has one element more than the simplified graph.

Given that these are unused links, we do not want to include them in our skims I think.

[jamiecook]

wouldn't it be better to not add links that go from a->a ?
also just wondering how you can know that [:-1] will catch all such cases...

[pedrocamargo]

The thing is that we set node_b = node_a for all links that are not part of that particular graph, and are not in the graph used for computing paths. This is done for managing super networks in multi-class assignments, as that procedure guarantees the order of the links in the graphs for all modes/classes.
The solution fixes it because the +1 that was introduced is for the Forward-Star that indexes links in the graph with everything.