Comment 2 for bug 594581

By the way, there are several other modularity variants that all differ in the null model being used; for instance, a directed modularity measure was proposed by Kim et al based on using the PageRank vectors instead of the degrees. There's also a Laplacian formulation of modularity by Lambiotte, Delvenne and Barahona: http://arxiv.org/abs/0812.1770. We might want to adapt the modularity function completely so it can take different null models.