Joint factors for deciding between multiple gp mentions

[Colossus]

I implemented a joint factor for this purpose, but it fails on two ddlog issues currently:

• NOT doesn't work; the exclamation mark isn't parsed in factors
• There is not way to do array indexing (for word index comparisons) in DDLog

[ThomasPalomares]

the "not" issue seems to be fixed with the https://github.com/HazyResearch/ddlog/pull/80 PR in ddlog. Hopefully the array indexing should be fixed soon !

[Colossus]

Work on 12647363_Abstract.0_1_50_12647363_Abstract.0_1_16-17-18