Cleanup/Clarification on Observable expression handling

Original title: Follow-up from "Resolve "TQMultiObservable should lazily evaluate subObservables""

The following discussion from !308 (merged) should be addressed:

• @rgugel started a discussion:

  @cburgard Do you see a reason why we (originally) called "compileExpression" twice (once before, once after parseExpression)? I essentially remove the second call for now as compileExpression replaces curly brackets by round ones (which renders the lambda expression invalid). In principle we could add those only after the (second) compilation but if it's not even needed if might be worth getting rid of it all together...