AP_VisualOdom: allow for visual odom position without yaw

[tridge]

the quaternion passed in via handle_vision_position_estimate is only used by EKF3 for yaw, and the EKF code handles the case that the quaternion is NaN. We need to accept NaN attitude to allow for external navigation systems that only provide a position. This requires that users setup the yaw source as compass

[rmackay9]

In general this seems OK but one potential issue and one thought:

• Besides the EKF, the yaw is also used in the AP_VisualOdom's yaw and position alignment code so this should be checked to confirm it handles the NaN correctly. This is important for GPS/Non-GPS transitions where we may re-align the external nav source to match the GPS and compass (or whatever the yaw source is when ExtNav is not used)
• If the EK3_SRCx_YAW param value is set to any value other than ExtNav (6?) then it shouldn't consume the yaw. It makes sense to have protections of course so that if an invalid yaw is passed in, it is not used.