kpvs: fix naming and inconsistencies

- Change Takeoff/Landing to Liftoff/Touchdown for Pitch/Roll.
- Updated names to match naming convention.
- Unit test fixes:
  - Implemented some of the .can_operate() tests.
- Added units to the KPV definitions.
- General tidying up of relevant nodes and test cases.
  - Strip excess docstring placeholders.
Merge branch 'master' of github.com:FlightDataServices/FlightDataAnalyzer