Add set serialisation & check for lists in attrs

[brynpickering]

Fixes issue(s) #461

Summary of changes in this pull request:

• Extend the data types that can be included in xarray attributes to include sets, which are serialised as lists.
• Extend attribute serialisation to those attached to data variables, not just those at the top-level of the xarray dataset.

Reviewer checklist:

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

[sjpfenninger]

Good to go
EDIT: though the tests actually fail, so maybe not

[codecov]

Codecov Report

Merging #489 (dae1bfe) into main (cbed4c8) will decrease coverage by 2.82%.
Report is 2 commits behind head on main.
The diff coverage is 95.87%.

@@            Coverage Diff             @@
##             main     #489      +/-   ##
==========================================
- Coverage   94.98%   92.16%   -2.82%     
==========================================
  Files          31       32       +1     
  Lines        3890     3996     +106     
  Branches        0      955     +955     
==========================================
- Hits         3695     3683      -12     
- Misses        195      196       +1     
- Partials        0      117     +117     

Files	Coverage Δ
src/calliope/__init__.py	100.00% <ø> (ø)
src/calliope/_version.py	100.00% <ø> (ø)
src/calliope/backend/helper_functions.py	99.35% <ø> (ø)
src/calliope/backend/where_parser.py	98.43% <ø> (ø)
src/calliope/cli.py	81.67% <100.00%> (ø)
src/calliope/core/__init__.py	100.00% <ø> (ø)
src/calliope/core/attrdict.py	99.50% <ø> (ø)
src/calliope/core/io.py	91.75% <100.00%> (ø)
src/calliope/core/model.py	93.23% <100.00%> (ø)
src/calliope/core/util/generate_runs.py	87.09% <ø> (ø)
... and 22 more