Improve droplet tracker by reusing data

[david-zwicker]

DropletTracker that determine droplets using refine=True should in principle be able to accelerate the fitting by supplying the droplets from the previous time frame. This would require changing some infrastructure to allow refining droplets using a list of close pre-defined droplets, but it might be worthwhile to implement that as an option. In any case, this feature must be optional, since droplet trackers with a long interval could otherwise run into trouble.