QGroundControl flight plan with fixed angle for multirotor.
Program modifies .plan Flight Plan file, saved from QGroundControl
related issue: mavlink/qgroundcontrol#7140
related discussion: https://discuss.px4.io/t/heading-fixed-flight-path-for-multicopters/1591/21
Usage: python plan_fix_heading.py <filename> <angle>
Generates <filename_fixed> flight plan
Angle, if ommited - will be taken from survey parameter Angle, if any