tracker/nn: Add support for 3x3 pos/size cov matrix (and some cleanup)

[DaWelter]

This PRs main purpose is to add support for 3x3 covariance matrices for position+size estimation.
My recent (unreleased) models output those instead of only component wise standard deviations.

Overall

• Accept 3x3 covariance matrix output for position+size
• Old models still worked fine in my testing
• Delete commented code which was there previously to support recurrent models. (Won't be used in foreseeable future)
• Delete leftover code for experiments with closed eyes detection (Likewise, I have given up on that / Is not really needed any more)
• Improve comments and code quality in the internal filter code

[sthalik]

👍

[sthalik]

As far as the unitialized metadata version field is concerned, could you add a custom metadata field to new models? Then the value would signify that the metadata isn't left uninitialized.

[DaWelter]

As far as the unitialized metadata version field is concerned, could you add a custom metadata field to new models? Then the value would signify that the metadata isn't left uninitialized.

Good idea. I could try this next time.