Merge pull request #337 from sjwilks/hott-esc

Driver for the reading Graupner HoTT sensors (such as their ESC range) via the telemetry port
Removed unneccesary casts in airspeed calculation to double precision
Cleanup comments and make them more consistent between messages.
Hotfix: Fixed RC calibration
Merge remote-tracking branch 'upstream/master' into hott-esc