remove default for waypoint speed

[DanAlbert]

FlyingGroup.add_waypoint allows a default speed of 600kph ground speed, regardless of aircraft type or altitude. There isn't a really a safe default, so this just invites programming errors that aren't apparent until runtime.

Instead, there should be two different functions: add_waypoint_with_speed and add_waypoint_with_tot (DCS requires that exactly one of those is set) where the argument is required.