Query improvements in the min cost flow algorithms.

- External flow and potential maps can be used.
- New query functions: flow() and potential().
- CycleCanceling also provides dual solution (node potentials).
- Doc improvements.

 @nodeset

 label

 0

 1

 2

 3

 4

 5

 6

 @edgeset

         weight

 0   3   1

 0   4   1

 1   5   1

 5   2   1

 6   2   1

 0   1   1

 2   1   1

 3   4   1

 4   5   1

 5   6   1

 @end