Add frontend-control for configuring parallel execution in backend

This issue is to plan the frontend-integration of the backend's parallel execution features (utopia#27 (closed), utopia!98 (merged)).

With utopya outsourced, the main aim is to communicate to any model implementation whether it makes sense to activate parallel features.

Proposal

• Provide CLI arguments to manually enable/disable parallel execution
• Consider providing information about whether parallel execution is available to the frontend
  • See this note in !98
• For sweeps, allow to decide whether there should be parallel execution in the backend or frontend-parallelisation
  • This could be made dependent on the number of WorkerManager worker tasks configured
• Evaluate what needs to be done for cluster support