Add multi-level sources to update_levels_from_other

[znicholls]

Description

Closes #26

Checklist

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Changelog item added to changelog/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.25%. Comparing base (e419123) to head (661b097).
⚠️ Report is 77 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   97.24%   97.25%   +0.01%     
==========================================
  Files          30       30              
  Lines        1489     1459      -30     
  Branches      170      164       -6     
==========================================
- Hits         1448     1419      -29     
  Misses         23       23              
+ Partials       18       17       -1     

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