Test different time resolutions

[brynpickering]

Fixes #354

Or, rather, it confirms my expectation that #354 is now fixed.

The model is agnostic of the timestep resolutions of the input data. It will just have NaNs where timesteps don't match.
Resampling then squashes all those NaNs out, so long as the resampling frequency is <= any of the input resolutions.

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 ✅

Comparison is base (e2bfb1a) 95.66% compared to head (519658a) 95.70%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
+ Coverage   95.66%   95.70%   +0.04%     
==========================================
  Files          24       24              
  Lines        3526     3562      +36     
  Branches      765      710      -55     
==========================================
+ Hits         3373     3409      +36     
  Misses         87       87              
  Partials       66       66              

Files	Coverage Δ
src/calliope/backend/backend_model.py	97.84% <100.00%> (+0.10%)	⬆️
src/calliope/backend/pyomo_backend_model.py	94.94% <100.00%> (+0.35%)	⬆️
src/calliope/cli.py	81.67% <100.00%> (ø)
src/calliope/examples.py	100.00% <100.00%> (ø)
src/calliope/model.py	93.65% <ø> (ø)
src/calliope/preprocess/model_data.py	100.00% <ø> (ø)
src/calliope/util/logging.py	93.87% <ø> (ø)