SMT-LIB: Parsing of models

[marcogario]

According to the standard (http://smtlib.cs.uiowa.edu/papers/smt-lib-reference-v2.5-r2015-05-28.pdf ) (get-model) should return a list of definition of the form define-fun.

We ignore this, because our get-model simply calls get-value for all variables defined in the formula. We should support reading the model with get-model. See work by Liana H on https://github.com/SMT-COMP/postprocessors/tree/master/model-validation-track