AP_Camera: changed update_location() to return true for camera trigger

Makes the main vehicle code responsible for taking a photo so logging can occur
Copter: trigger + logging on camera update_location()
Plane: trigger picture + logging on camera update_location()
AP_Camera: removed incorrect mavlink sends

these sends did nothing, as MAVLINK_COMM_3 is not connected to
anything. If it was connected they would be dangerous, as a blocking
serial write could cause the stabilization of a copter not to run. If
the serial port was non-blocking it would corrupt the packet.

If we needed something like this it would have to use the MAVLink
packet queueing logic we use elsewhere
Plane: use current_loc.alt for altitude to TECS
Plane: allow enable/disable of camera log messages in CLI
Copter: enable camera logging by default

these messages only happen on user defined camera triggering, so
should be low rate
AutoTest: fix to copter getting stuck in Failsafe test
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
AP_Baro: added GND_ALT_OFFSET parameter

used for automatic barometric adjustment by a ground station equipped
with a barometer
Plane: fixed a documentation string

GND_ALT_OFFSET is now for barometric corrections
AHRS: typo fix for ORIENTATION param description