Prioritized optimization

[Broes De Cat]

That is, performing optimization first on my top priority and getting optimal solutions, then among those solutions performing optimization on my second top priority and getting possibly fewer optimal solutions, and so on.

For instance, the hotel booking problem, which isExample 3.11 on Page 22 of the gringo/clasp manual here:

http://www.cs.utexas.edu/~vl/teaching/lbai/clingo_guide.pdf

[Broes De Cat]

Untested support has been added to the solver, should add a new Inference to idp to get it running.

[Broes De Cat]

Removing milestone: Later (automated comment)