Add tests and functionality for loading provenance data

[h-mayorquin]

Description

This PR adds the functionality for loading the provenance data to a Labels object. Most of it was already in place and there was just the initialization missing.

I also added a test. As the old data fixtures did not seem to contain provenance data (they have empty dicts) I also generated some fixture by running your software latest version in your training data tutorial. This maybe should be changed as I don't know what's your policy for adding fixtures and this was meant to be temporal.

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (explain)

Does this address any currently open issues?

No

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP-IO!

❤️

[codecov]

Codecov Report

Merging #14 (1585a9d) into main (00c2125) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #14   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          515       517    +2     
=========================================
+ Hits           515       517    +2     

Impacted Files	Coverage Δ
sleap_io/io/slp.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[talmo]

This also looks good to me! Policy for adding fixtures is to keep it to a minimum, but this PR is quite helpful actually -- thanks!