Added Slat Excluding/Including Transition.
Flap as used by KPVs is always a multi-state.

- Fix old assumptions about state names in unit tests.
- Remove non-multi-state code from helper functions.
- Ensure that state names are not munged anywhere - always strings.
- Fix outdated docstrings and documentation.
Airspeed With Flap * 0 And Slat Extended Max

For safety events we have KPVs using 'Flap Lever' or the synthetic
fallback but for maintenance events that use the derived parameters
that relate to the flap surface we need an extra event and KPV to handle
the special retracted flap with extended slat case.
Merge branch 'master' of github.com:FlightDataServices/FlightDataAnalyzer