AP_NavEKF3: update core selection logic to prevent non-aiding core preference

[kush-marble]

Summary

Update the core selection to choose cores that are aiding and fusing data over cores that are not.

Classification & Testing

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Tested in multiple quadplane flights.

Description

In a recent flight I had an issue where a core was selected with a bad GPS (affinity was enabled). This caused the aircraft to go into DCM. Looking at the log data it showed that core 1 had a constant error score of 0; because it was not fusing any data so there were 0 innovations etc. This core should not have been chosen over core 2, that was fusing data from the good GPS (like core 0 was).