[modularity] Handling multi-graphs

[luccitan]

The current implementation is based on Gephi's considerations.
Thus, for a directed graph, it looks for a <-- b during the a --> b, handling Gephi's considerations in an iteration-through-edges way.
If a graph is multi, searching for a <- b edge during the a <-> b iteration is then possible but leads to weird behaviour.

See the source code and the upper note for a better understanding.