Compliance with new CIVL IGC draft
https://drive.google.com/open?id=0B2nArJ8E2TB5SXJ2M0Y0bFV6b3BfZUQxTmVWZ2VnemZFWmww
- Add duplicate HFDTE/HFDATE to allow backwards compatibility
- Add 20s buffer before takeoff
- Add support to record baro height even when GPS fix is lost