Track fusion edit action

[cmalinmayor]

Idea: Allows you to merge parallel tracks that are oversegmented, essentially. If you select two nodes in the same time point, you can click "merge tracks" and it will merge the tracks from that time point onward, combining the segmentations and deleting the second track.
If you have no segmentation, you can still do this and it will average the positions of the detections in each frame.

Implementation Actions:

• Delete the second track (but keep seg/position information)
• Update segmentation/position of nodes in first track