Extend P-log with additional type of queries (and possibly new syntax).

[iensen]

Currently, we only allow queries of the form ?-a that return the answer.
It can be useful to have other type of queries, such as MAP inference, Probabilistic Plannning (MDP/POMDP, Influence Diagrams), etc. Additional changes to P-log syntax may be required.

[shiqizhang6]

@iensen Good to see you bringing up the MDP/POMDP syntax here. Hope I can help here somehow.