Add prograde and retrograde for surface velocity

[jbru]

Currently the prograde and retrograde are only available for orbit velocity, you can't for example set steering to the surface retrograde of a planet to kill the relative velocity for landing.

The current implementation uses Vessel.orbit.GetVel() for the orbit velocity, you can just clone both keywords and use Vessel.GetSrfVelocity() for the surface velocity.

[Dunbaratu]

You can set steering to a vector. And while there aren't Euler rotations for surface prograde and retrograde, there ARE velocity vectors for it.

This for example, works:

lock steering to mySteer.
set mySteer to R(0,0,0) + velocity:surface.

If scalar times vector finally works now in 0.85, then you should be able to get the retrograde by doing this:

set mySteer to R(0,0,0) + ( velocity:surface * (0-1) ).

(multiply surface velocity by scalar negative 1).

This isn't a good enough reason to avoid putting the Euler rotations in there as well, but it can work as a temporary workaround until it's there.