added MPLP

Max-product LP relaxation inference for factor graph model.
Unit-test has been added.
Examples will be added soon.
@iglesias hushell
I agree to merge `MDArray` into `SGNDArray`. Thanks for your discussion above, now I see it is possible to implement them in just one class. I am currently re-factoring MPLP. Once refactor work is done, I will put MDArray into SGNDArray as soon as possible.