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

 echo '#include<'$1'>' >seedeps_tmp.cc

 gcc -MM -I. -I.. -I../.. $2 seedeps_tmp.cc|tr ':' ' '|tr '\' ' '|tr ' ' '\n'|

 egrep -v '^$'|egrep -v '.o$'|egrep -v '.cc$'|grep -v '^'$1'$'|sort

 rm seedeps_tmp.cc