Copter/Rover: vision-position support should be moved to AP_VisualOdom

[rmackay9]

Copter-3.6 and Rover-3.3 can consume these external position estimate messages:

• ATT_POS_MOCAP
• VISION_POSITION_ESTIMATE
• VICON_POSITION_ESTIMATE
• GLOBAL_VISION_POSITION_ESTIMATE

These are then passed into the EKF by GCS_MAVLink library but this functionality should be moved to the AP_VisualOdometry library because:

• we need the camera offset position which is held in AP_VisualOdometry
• the messages are quite similar

This would also allow us to add a parameter to control whether the message's system time is used or the flight controller's time (held in the message) should be used.

[rmackay9]

I think @peterbarker says he'll take this on

[rmackay9]

This has been done now in master so closing, thanks!