Redesign track kf

We will create tracks on keyframes on demand. We will remove all structures explicitly storing tracks on keyframes.

In particular, TrackMatrix::tracks_kf_ needs to disappear. Also the related API.

The only remaining fcn is tracksAtKeyframes() which is reimplemented to assemble track_kf from track on demand.

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• 1b479aba - Remove all traces of tracks_kf_

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mentioned in commit 4db0d6f8