Add `maxCostFlow` function to the `MinCostFlow` module

[toyboot4e]

It's super handy.

[toyboot4e]

No, seems like it's difficult to generalize it with Dijkstra-based min cost flow.

[toyboot4e]

It was impossible with Dijkstra; user have to handle potencials by themselves.

• Bellman-Ford (O(nm)): possible
• Dijkstra (O((n+m)\log n): possible, but requires manualy potencial handling