Add new module for flow based connectivity and cut algorithms

With the addition of the function "ford_fulkerson_flow_and_auxiliary" it is now possible to easily access the residual/auxiliary graph resulting of the computation of a maximum flow.

This new module implements flow based connectivity and cut algorithms for both directed and undirected graphs. This pull requests contains also tests and documentation.

I also have a few more nice flow based cut algorithms in the pipeline. They depend on the functions in this new module and are quite more complex, so to keep it simple I would first add the basic functions contained in this pull request. I'll add the more complex ones when/if this is merged.