Add a isOpen and isClosed function to generated actor API

[Bethibande]

Task

• Add isOpen and isClosed functions to the generated API, these functions must not send commands but simply delegate the result of the AbstractActors isOpen / isClosed function.

Motivation

A way to check if it is still possibly to interact with an actor. Sending a command to a closed actor will cause an IllegalStateException.

Notes

• depends on #8, add these functions to the new API super-class directly

[Bethibande]

Has been implemented in 70e39a03451ffd7ad24ebfdbc0fd56d4b33d4c3d because of #8