[Feature Request] Functions to calculate modularity on all available graph models

[LibraChris]

Is your feature request related to a problem? Please describe. Modularity is a very important measure of the graph structure. The Louvain method for community detection operates on modularity optimization, yet a simple method to calculate the modularity of a graph outside of the louvain methode is not implemented.

Describe the solution you'd like Both graph models supported by FSharp.FGL should include a methode for modulartiy calculation.