Add record of single element attribute lists in IO

[brynpickering]

Fixes #614

Summary of changes in this pull request

• single element lists / sets are logged and returned to being lists / sets on loading from NetCDF

Reviewer checklist

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.35%. Comparing base (872978d) to head (5281200).
Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #627      +/-   ##
==========================================
- Coverage   95.85%   95.35%   -0.51%     
==========================================
  Files          24       24              
  Lines        3619     3656      +37     
  Branches      788      734      -54     
==========================================
+ Hits         3469     3486      +17     
+ Misses         86       82       -4     
- Partials       64       88      +24     

Files with missing lines	Coverage Δ
src/calliope/io.py	96.87% <100.00%> (+2.08%)	⬆️

... and 3 files with indirect coverage changes

[irm-codebase]

Looks good overall. I just have one question about whether or not the fix can be made more generic.
After that I can approve.