ENH: add Mission.trajectories_3d() and Mission.positions()

Wraps CompareFlights(mission.all_flights) directly on Mission so a
caller doesn't need to know CompareFlights exists just to plot a
mission's trajectory - matches how a single Flight already exposes
its own trajectory_3d(). all_flights stays available for any other
CompareFlights plot these two don't cover.

Author: aZira371