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.

src/processor/processor_tracker_feature.cpp

@@ -147,7 +147,6 @@ void ProcessorTrackerFeature::establishFactors()
     if (origin_ptr_ == last_ptr_)
         return;
 
-    track_matrix_.markKeyframe(last_ptr_);
     TrackMatches matches_origin_last = track_matrix_.matches(origin_ptr_, last_ptr_);
 
     for (auto const & pair_trkid_pair : matches_origin_last)