Added possibility to set heading through flight plan for rotorcraft

I was surprised there did not seem to be a good way to change the heading through the flight plan for rotorcraft, so I added a small function. With NavHeading you can now set your heading to a fixed value and with set_nav_heading_by_waypoint it can be set to aim at a waypoint. As a workaround I used an exception to have the function called continuously while using go to fly to a waypoint. I couldn't find another way to have a function called while using go or stay.
I hope this is useful