Implement cutoff and fix value_only use in minimum_cut

I only added cutoff to `edmonds_karp` and `shortest_augmenting_path`. I suppose that I also know how to add this to `preflow_push`, but I cannot make sense of the residual network produced when both `value_only` is `True`, and `cutoff` is smaller than the maximum flow value. (To summarize, it represents a preflow that is not maximum; there is not *s*–*t* paths, but the induced *s*–*t* cut is not minimum. But then what?)

I also fixed `value_only` use in `minimum_cut`. It should be `True` when passed to the underlying maximum flow algorithm.