Add `Track` when add `Instance`

[roomrys]

Description

Previously, the Labels.add_instance method would add the track associated with the instance to the cache, but not to the Labels object itself. I did not verify this, but I am sure this is handled in the call sequence when SLEAP adds tracks; however, it makes the SLEAP API a bit difficult to use (leading to KeyErrors in the LabelsDataCache dictionaries).

This PR adds the Track associated with an Instance to the Labels.tracks list when calling Labels.add_instance.

Types of changes

• [ ] Bugfix
• [ ] New feature
• [ ] Refactor / Code style update (no logical changes)
• [ ] Build / CI changes
• [ ] Documentation Update
• [x] Other (better API)

Does this address any currently open issues?

[list open issues here]

Outside contributors checklist

• [ ] Review the guidelines for contributing to this repository
• [ ] Read and sign the CLA and add yourself to the authors list
• [ ] Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• [ ] Add tests that prove your fix is effective or that your feature works
• [ ] Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

:heart:

[codecov[bot]]

Codecov Report

Merging #1408 (bcefd05) into develop (d173303) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1408   +/-   ##
========================================
  Coverage    72.74%   72.74%           
========================================
  Files          133      133           
  Lines        23723    23722    -1     
========================================
  Hits         17257    17257           
+ Misses        6466     6465    -1     

Files Changed	Coverage Δ
sleap/io/dataset.py	83.66% <100.00%> (+0.08%)	:arrow_up:

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