MaxCut should not implement Graph

[GiggleLiu]

Meanwhile, why both compute_cut_weight and cut_size?

[isPANN]

Btw, should we maintain a category for each problem?

[GiggleLiu]

I do not think so, this is added by AI directly, I do not know whether it can be useful. But I think a distinction between OptimizationProblem and SatisfactionProblem should be made. A refactoring is in #50 , feel free to add more refactoring suggestions.

[GiggleLiu]

Both fixed by #50