Changes based on new version of ProblemReductions

[c-allergic]

This pull request aims to utilize the interfaces inProblemReductions to simplify the package.

Updates

1. The solution size of Coloring/Satisfiability is now defined as the number of violations of colors/clauses. The smaller the better now.
2. Rename best_solutions to largest_solutions, best2_solutions to largest2_solutions and bestk_solutions to largestk_solutions.
3. Remove the weights from PaintShop.
4. Remove the weights on vertices from MaxCut.
5. SpinGlass is no longer specified by cliques. It is now specified by graphs or hypergraphs. Weights can be defined on both edges and vertices.
6. Remove unit_disk_graph, replace it with UnitDiskGraph from ProblemReductions.
7. variables always return 1 to n, where n is the number of variables. Use symbols to access variable names in an SAT.

[c-allergic]

TODO:

1. Deprecate the GraphProblem, chweight and ZeroWeight.
2. Fix test dependency

[GiggleLiu]

Good job!