Use waypoint altitude for all turnpoints

[Joerg Ewald]

Right now, waypoint altitude is not used for turn points. The only altitude being set is goal altitude, which was used until now for stopped tasks. But with CESS and AATB, we need to know altitudes of other turnpoints as well.

Solution: Store waypoint altitude for each turnpoint, use that for all altitude-relevant calculations. Remove goal altitude field from task definition.