AP_VisualOdom/Scripting: support Odometry's quality, add autoswitching script

[rmackay9]

This improves the safety of using the ModalAI VOXL sensor by adding support for automatic switching to optical flow when the sensor loses its position estimate.

The detailed changes are:

1. the ODOMETRY mavlink message's quality field (which the ModalAI provides) is consumed, and logged
2. a new VISO_QUAL_MIN parameter has been added which blocks sending of positions and velocities to the EKF if their quality is too low. This parameter's default is zero so when ModalAI failed (and quality becomes -1) we will not send to the EKF. Other cameras are not affected because the quality will be 0 (unknown) and will thus pass and be sent to the EKF.
3. a new ahrs-source-extnav-optflow.lua scripting applet has been added which allows easy manual or automatic switching between external nav and optical flow
4. SITL's vicon feature sends a quality of 50 and gets parameter documentation

This has been tested in SITL and on a real vehicle and greatly improves handling of ModalAI camera failures.

Below is a screen-shot showing that SITL is sending a quality of 50.

Below is a screen shot of a test to ensure the VISO_QUAL_MIN was working. When the required quality was increased above 50, SITL's simulated vicon values were not fused, the EKF failsafe triggered and the vehicle switched to Land mode