Refactor model builder from model engine to quant engine.  Work on fairness strategy for CEGQI.  Add option for single/multi triggers.  Minor cleanup.
Model building into quantifiers engine.  Simplify axiom-inst mode.
CEGQI uses model.  Enforce fairness in CEGQI natively.
trace decision-node
amend prvs commit
Context-dependent expr attributes are now attached to a specific SmtEngine, and the SAT context is owned by the SmtEngine.