Allow configuration of output channel for --dump-models

[bclement-ocp]

Alt-Ergo uses the standard input to print models. This is in accordance to the SMT-LIB standard.

However, while this makes sense for the models that are printed as a result of a (get-model) command from the user, it makes less sense for the models printed by the --dump-models option. We should consider printing those to stderr by default, and bring back an option to let the user control this output channel independently.

(Seel also #393)

[Halbaroth]

I guess this issue is solved?

[bclement-ocp]

Yes fixed in #838