Unit Propagation Over Grounded MRF

@ashish33 @niranjanb 

Add unit propagation over the grounded MRF. Command-line option to enable is "-unitPropagate". Works in conjunction with "-writeClauses <file>" option to write reduced MRF to file.

Currently unit propagation only works for non-partitioned inference, so also needs to have the "-nopart" option enabled.

On larger MLNs there are some differences in the marginals produced by Tuffy with and without unit propagation. I haven't been able to find an error in the unit propagation implementation, but there may be one. Running Tuffy with the "-verbose 3" option will, among other things, print out each step of unit propagation as it executes.
Should we not raise a flag/exception and stop here if the unsatisfiable cee is a hard clause?