add bounding method in factor graph model for submodularity

- added bounding method for solving submodularity issue in graph cuts (currently only PrimalMosekSOSVM supports this feature)
- updated multilabel classification example
- solved hinge loss < 0 issue in SOSVMHelper
FactorGraphModel should not be tied with GraphCut. Maybe check inference type? And call a special set up of bounds, e.g., ```CMAPInference::set_learning_constraints()```, something like this.