Improvements and unifications for BellmanFord (#51)

- Rework the function type interface to fit to dijkstra().
- Rename named template parameters (Def* -> Set*).
- Rename some private member variables
(to start with an underscore).
- Simplify template parameter names.
- Many unifications and improvements in the doc.

 #define LEMON_VERSION "@PROJECT_VERSION@"

 #cmakedefine LEMON_HAVE_LONG_LONG 1

 #cmakedefine LEMON_HAVE_LP 1

 #cmakedefine LEMON_HAVE_MIP 1

 #cmakedefine LEMON_HAVE_GLPK 1

 #cmakedefine LEMON_HAVE_CPLEX 1

 #cmakedefine LEMON_HAVE_CLP 1

 #cmakedefine LEMON_HAVE_CBC 1