added MPLP

Max-product LP relaxation inference for factor graph model.
Unit-test has been added.
Examples will be added soon.
Hey @iglesias We do need higher dimensional arrays to store messages involving 3 or more variables. It's possible to flatten the arrays, but then base sizes have to be recorded, which is like reinvent a multi-dim-array class. How about we keep Jiaolong's implementation here? And we open an TODO issue for future merging of these two classes?