Support tolerance technique for BellmanFord (#51)

A new operation traits class BellmanFordToleranceOperationTraits
is introduced, which uses the tolerance technique in its less()
function. This class can be used with the SetOperationTraits
named template parameter.

#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