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.

 EXTRA_DIST += \

 	tools/Makefile

 if WANT_TOOLS

 bin_PROGRAMS += \

 	tools/dim_to_lgf \

 	tools/lgf-gen

 endif WANT_TOOLS

 tools_lgf_gen_SOURCES = tools/lgf-gen.cc

 tools_dim_to_lgf_SOURCES = tools/dim_to_lgf.cc