Print response to get-model using the API (#7084)

Print response to get-model using the API (#7084)

This changes our implementation of GetModelCommand so that we use the API to print the model.

It simplifies smt::Model so that this is a pretty printing utility, and not a layer on top of TheoryModel.

It adds getModel as an API method for returning the string representation of the model, analogous to our current support for getProof.

This eliminates the last call to getSmtEngine() from the command layer.