Support min. cost max. flow in MCF classes

[Peter Kovacs]

The new concept of the min cost flow classes (see #234) makes it easy to provide interface for the min. cost maximum flow problem, too.

E.g. there could be a maxFlow(Node s, Node t) function, which could be used instead of supplyMap() and stSupply(). In this case Preflow::runMinCut() should be called to determine the max. flow value (instead of Circualtion), and the algorithm have to be initialized as if stSupport() was called with this flow value. However apart from that nothing have to be changed.