GPS: added set_secondary() function

for preventing notify updates for 2nd GPS
Plane: set 2nd GPS as secondary
Copter: set 2nd GPS as secondary
Rover: set 2nd GPS as secondary
Flymaple: RCOUtput disable_ch() now supports disabling outputs
AntennaTracker: now supports mavlink arm and disarm commands to disable antenna servo outputs
AntennaTracker: finish adding GPS support, including using relative or absolute altitudes
AntennaTracker: use gcs[] array instead of old idiom gcs0, gcs3
AntennaTracker: add support for MANUAL and AUTO modes
AntennaTracker: initial documentation
AntennaTracker: update documentation
AntennaTracker: Improvements to yaw slewing and ballerina mode
Frame_params: Updated tilt angles for Tarot Gimbals
AP_NavEKF : Fix bug that leaves height unconstrained in static mode
AP_TECS : Add protection for airspeed runaway due to pitch errors
AP_NavEKF : Add low speed magnetometer calibration option
AP_NavEKF : Add sideslip constraint to allow plane flight without magnetometer
AP_NavEKF : update comments
AP_NavEKF : Make all compass use autodetect
AP_NavEKF : enable operation without a compass for planes