Improvement: allow CKF to keep stopped branch

[timadye]

Following on from #2905, @andiwand suggested:

Maybe the branch stopper can also return if the track should be kept or not? I could imagine that we could use both scenarios: early removal of uninteresting tracks and stopping the propagation to save CPU and keeping the tracks.

Maybe instead of the CKF's BranchStopper returning a bool, it could return an:

enum BranchStopperResult {
  ContinueBranch = 0,
  DiscardBranch = 1,
  StopAndKeepBranch = 2
};

Although the enum and bool values are upwardly compatible, the type is obviously different, so I guess this would require a breaking change.

If we are already changing the API, it might be useful to simplify the call signature to pass the track state by const &:

  using BranchStopper = Delegate<TrackStateFlag(const CombinatorialKalmanFilterTipState&,
                                      const typename traj_t::TrackStateProxy&)>;

This proposal is not urgent, so it could be included with the next breaking release.

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[AJPfleger]

@andiwand @timadye what is the status on this?

[andiwand]

this is handled with #3102

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