Flow algorithms improvements

[afourmy]

As of now, four algorithms are implemented to find the maximum flow. All four of them compute the maximum flow from a source node to a destination node (aka sink node).

Allow the user to define not just one source / sink node, but a set of sources / sinks, and update the algorithm so that it adds a virtual (dummy) source (resp sink) when there is more than one source (resp sink).

Same with minimum-cost flow.