Refactor RunModel to use threading.Event for cancellation
This commit changes RunModel and EnsembleEvaluator to use threading.Event
to cancel instead of queue.SimpleQueue.
Ideally, we would use asyncio.Event(), but that caused a lot of issues
in everest.
Fix backend handling in the optimization config
- Fixes handling of the backend keyword.
- Added a test for this.
- Small fix of a method name in everest2ropt.
- Fix deprecate usage of "backend" in two tests.