Harmonize expression treatment in TQObservable::getObservable() and TQObservable::Manager::createObservable()

Currently getObservable calls createObservable with the expression that was originally passed to getObservable (if it couldn't find an existing observable). However, getObservable compiles the expression at the beginning of the function and uses the compiled expression or derivatives thereof for its own steps. createObservable, however, does not perform this compilation. The line for this in createObservable is commented (as it breaks the observable matching, e.g., in the HWW setup, resulting in a runtime error). However, the line in createObservable is slightly different from the call in getObservable (with respect to replacing/unreplacing bools). A fully consistent treatment is highly desirable here.

TODO: Use the compillation call from getObservable in createObservable and test this.