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