[Aorai] Add a list of observables

- add an "observables" option at the begining of ya files
- multiple "observables" options merges together (union of symbols)
- check that every function referred to is inside the observable list
- remove the initial curOperation and curOpStatus : the main function is
  not necessarily observable and the pre/post function will set those
  variables anyway
- ignore unobservables functions inside the relevant visitors