AP_GPS: zero state structure on driver release

this ensures no values are left from the previous driver
AP_GPS: handle multiple GPS in GPS setHIL()
DataFlash: hdop is unsigned
Replay: handle multiple GPS sensors
Plane: update for GPS.setHIL() API
Rover: update for gps.setHIL() API
Copter: update for gps.setHIL() API
Ublox: Enable 1Hz Lock LED by default
Replay: fixed velocity vector