(Copter) surveys are freezing when slowdownEnabled="true"

[han16nah]

When slowdownEnabled is set to "true" (default for multicopter platform), the survey often does not finish but completely freezes at the end of a waypoint. It seems like the platform slows down so much that it never reaches the waypoint. The bug was likely introduced in 185a3fc. It can be reproduced running: helios data/surveys/voxels/uls_detailedVoxels_mode_comparison.xml (freezes in Leg1/11 at 100%)

(A current workaround to the issue is setting slowdownEnabled="false" in the survey-XML file)