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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.35%. Comparing base (872978d) to head (5281200). Report is 19 commits behind head on main.

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/calliope-project/calliope/pull/627?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project) | Coverage Δ | | |---|---|---| | [src/calliope/io.py](https://app.codecov.io/gh/calliope-project/calliope/pull/627?src=pr&el=tree&filepath=src%2Fcalliope%2Fio.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project#diff-c3JjL2NhbGxpb3BlL2lvLnB5) | `96.87% <100.00%> (+2.08%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/calliope-project/calliope/pull/627/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=calliope-project)