JOE should not allow language=shell for pre/postprocessing

Current Situation

• Job monitors for pre- and postprocessing are available for scripting languages. They are not available for shell scripts. Should the need arise to process shell commands before or after a job then this can be easily mapped to a job chain with the respective job nodes.
• JOE offers job monitors to be implemented with language="shell".

Desired Behavior

• JOE should not offer monitor implementation with language="shell" as this is not supported by JobScheduler.