Fixes as flap and slat are multi-states!

- Don't use raw array when working with Flap Lever:
  - Raw values may not be what you expect, e.g. 1 = 'Lever 0'
  - Multiple state names for clean configuration, e.g. '0', 'Lever 0'.
- Extended some test cases to check the above.
- Improved and added some docstrings.
- Fix 'Slat', 'Flap' and 'Flaperon' - use M() not P()!
- Fix test cases broken by past configuration changes.
- Fix test cases broken by past refactoring.