Plane: added height to 10Hz TECS call
TECS: added height update_pitch call
Plane: added relative_altitude helper functions

prevents some code duplication
AP_GPS: switch fields to add units suffix

altitude -> altitude_cm
ground_speed -> ground_speed_cm
ground_course -> ground_course_cd

this helps prevent unit mixups
AP_AHRS: changes for GPS field changes
DataFlash: changes for GPS field changes
Rover: changes for GPS field changes
Copter: changes for GPS field changes
Plane: changes for GPS field changes