pressureCurve implementation

Implemented pressureCurve data and useLegacyAtmosphere setting. If useLegacyAtmosphere is set to false then it will try to find a pressureCurve node in the applicable body. If one is found it will load it and replace the body's existing pressureCurve (which for stock seems to just be some linear 0-1 data)

If no pressureCurve data is found it will log a warning.

Also added is a 'watchdog' class that checks every time the flight scene is loaded to make sure that useLegacyAtmosphere is still set for the body the way it was configured. May not be necessary after all but could prove useful if we need to stop KSP changing settings on us behind our back.