Improvements in groups.dox (#188)

- Unify the notations used for formulas.
- Add 'namespace lemon {...}' to simplify the references.
- Improved doc for algorithm groups.
- Extend the doc of the "shortest path" and "minimum cost flow" modules.

 if WANT_TOOLS

 bin_PROGRAMS += \

 	tools/dimacs-to-lgf

 dist_bin_SCRIPTS += tools/lemon-0.x-to-1.x.sh

 endif WANT_TOOLS

 tools_dimacs_to_lgf_SOURCES = tools/dimacs-to-lgf.cc