Merge new api changes to xadd

[mike-gimelfarb]

• visualize dbn seems to work properly for me now
• the only thing i changed was that the superclass model cpfs dict now has the format name: (objects, expr) instead of name: expr.. this was necessary to ensure that both grounded and lifted models can be interchanged in some code.
• the objects are ignored for xadd because the domain is assumed grounded.
• the only file i made a nontrivial change is the RDDLModelXADD, all other changes are cosmetic.
• I have not checked the level analysis nor simulator, but I understand now this is not on the critical path.
• please take a look at it and fix anything i missed.