Add option to allow Waypoints with altitude.

[mgalyean]

Often it is simpler to target a point above the desired landing zone by a few thousand feet to more easily cancel horizontal velocity well above target and so drop straight down to landing zone. I think of the airborne waypoint much like a gate on a ski slope or a channel marker when boating. If you make the gate or marker you know you are on course. Stock waypoints allow an altitude that is above terrain altitude and Trajectories allows using Waypoints as the target, but the impact marker tracks the ground position, not the mid-air position, when the Waypoint has an altitude above ground. When coming in at a shallow angle, using the ground impact marker leads to sliding in very fast horizontally and trying to scrub that vel at the last moment and that is unnecessary if one zeroes well above the target instead.

[PiezPiedPy]

I like this idea, maybe one day I or someone else can implement it.

An option when selecting a waypoint for target could be implemented to allow the target to intersect at ground or waypoint height.