Flow and CTC conversion fix

[joris-gentinetta]

• In the track function, link() is called before add_flow(), such that the flow is never actually used.
• In the to_ctc function, the forward_map returned by relabel_sequential is modified like this: 'fw[-1] = -1', because the parent_id -1 (no parent) should be mapped on itself. However, instead of adding a map from -1 to -1, this modifies the last value in the map and -1 is mapped to the default of 0. As the CTC format expects 0 for no parent, this is not an issue, but through the modification of the map, the parent of the last tracklet is set to -1, effectively cutting it off.

[JoOkuma]

Hi, @joris-gentinetta, I appreciate the PR, and thanks for catching this.

We were mainly using the CLI for the CTC submission.

I left a minor comment, once you accept the change and tests pass I'll merge the PR.

[codecov]

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