AP_GPS: added have_vertical_velocity() function 

for use by EKF
Rover: removed unused variables

thanks to Mike McCauley
Plane: remove unused variables

thanks to Mike McCauley
AP_NavEKF : Prevents aliasing triggering innovation consistency check failures
AP_AHRS: Change airspeed_estimate to const in children of AP_AHRS

Allows roll/pitch controllers to use DCM's airspeed estimate. Thanks to Kevin Hester for assistance in finding this.