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.

 #!/bin/bash

 cd ${0%update-libstd-tag}../doc

 mv libstdc++.tag libstdc++.tag.old

 wget http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/libstdc++.tag &&

 rm libstdc++.tag.old || mv libstdc++.tag.old libstdc++.tag