isConsistent does not check function constraints

[Broes De Cat]

As a result, if e.g. the codomain of a constant c is empty but c occurs nowhere, modelexpansion returns models (of the rest of the theory) but in debug throws an error that the structure is not twovalued.

[Broes De Cat]

For example in the "recursionNegation" test, there shoul in fact be no models (although this is not the intended result of the test).