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.

Doc improvements for Bfs, Dfs, Dijkstra (#185)

- More precise references to overloaded member functions.
- Hide the doc of the traits class parameters.
- Better doc for named groups.
- More precise doc for the case of multiple sources in Dfs.

Merge