specificGasConstant support

- Fixed issue with Jool NaN temperature. (capped low end of
getExternalTemperature() to -160)
- Capped low end of ambientTemperature to absolute zero.
- NaN protection for part.temperature
- Added density field to debug GUI
- Replaced hard coded gas constant with per-planet specificGasConstant.
(to-do: move that data to config files)
- ReentryPhysics still uses hard coded 287.058 value
- Added flight event logging for parachute failures.