Refactor dataset manager

[gabegma]

Part of #297

Description:

• Refactor get_tags so the output is the same as the input format for add_tags. It felt odd to me that they were slightly different (list vs dict). Let me know if you disagree, I can change it back to a list.
  • I removed the pandas conversion and the method is now faster.
  • I refactored the test which was messy and didn't test much the get_tags method.
• Add a new method to convert persistent_id to row_idx. This will be useful for my next PR, where we modify the route to update the tags.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[JosephMarinier]

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