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
Yes, currently, only pairwise factor type is considered. I added `TODO` there is because `w_dim == 4` can be problematic. For example, for a unary factor with status space of 4.