Improve FractionalFlow implementation in allocation

[SouthEndMusic]

#1429 changes the implementation of FractionalFlow in allocation by giving every node that has fractional flow outneighbors a buffer for flow that is not directly used. This implementation is however not correct. Using buffers to let allocation stick to the FractionalFlow flow distribution can only work if there is a buffer for every FractionalFlow node. There is also the additional problem that flow from the buffer should take precedence over other sources, especially when collecting subnetwork demands.