modularity.get_modularity_sig returns community-level modularity

[rmarkello]

It would be great if we could have an optional parameter to toggle whether we're interested in testing the significance of the modularity for each communities or for all the communities. Sometimes we're more interested in the latter than the former, but currently only allow for the former.

This would be as simple as summing the modularity estimates for each of the permutations and testing against these sums rather than testing against the arrays of modularity estimates (one estimate per community)