Coerce model dimension dtypes to numeric

[brynpickering]

Fixes #567

Summary of changes in this pull request:

• Always try to convert data dimensions to numeric and keep any successful attempts.
• Add some tests.
• Add mention of this in the docs.

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 95.86%. Comparing base (872978d) to head (a099c5b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
+ Coverage   95.85%   95.86%   +0.01%     
==========================================
  Files          24       24              
  Lines        3619     3629      +10     
  Branches      788      724      -64     
==========================================
+ Hits         3469     3479      +10     
  Misses         86       86              
  Partials       64       64              

Files	Coverage Δ
src/calliope/preprocess/model_data.py	100.00% <100.00%> (ø)