Add operate mode

[brynpickering]

Fixes #432

This is mostly to confirm that we're happy with how we approach adding new run modes into the new calliope internals.
I would love to have operate mode work with #127 but suspect it needs more planning.

Summary of changes in this pull request:

• Add a slim-line operate mode back into the model
• Switch from defining operate window/horizon as timesteps to being pandas time frequencies

Reviewer checklist:

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

[brynpickering]

Worth noting that in 4991efe I implemented it in the backend, then I switched it to being implemented in model.py. I think it works better as a "controller" outside the backend, but maybe not polluting model.py? Imagine adding spores mode to the same class and then you start to see calliope.Model becoming quite large. Maybe that's ok though

[brynpickering]

Options for run modes:

1. No run modes at all. Then we provide these different modes as external scripts. Those scripts might be a bit messy because we should avoid accessing private attributes / methods of the Model class, but it might make it cleaner to add new modes.
2. Everything added to the Model class (as is currently the case). Could bloat the Model class.
3. Everything added to the BackendModel class (as I implemented in 4991efe). Leads to needing to re-initialise "self" which is messy.
4. Everything added to some intermediate module (stored in backend?) that acts as a controller. This is akin to run.py in the v0.6 implementation.

[sjpfenninger]

Or introduce these operate mode quirks in the docs

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (60b0df8) 95.65% compared to head (46678a8) 95.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #539   +/-   ##
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  Coverage   95.65%   95.66%           
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  Files          25       24    -1     
  Lines        3450     3526   +76     
  Branches      747      700   -47     
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+ Hits         3300     3373   +73     
- Misses         85       87    +2     
- Partials       65       66    +1     

Files	Coverage Δ
src/calliope/__init__.py	100.00% <100.00%> (ø)
src/calliope/attrdict.py	96.50% <100.00%> (+0.01%)	⬆️
src/calliope/backend/backend_model.py	97.73% <100.00%> (+0.06%)	⬆️
src/calliope/backend/pyomo_backend_model.py	94.58% <100.00%> (+0.06%)	⬆️
src/calliope/examples.py	100.00% <100.00%> (ø)
src/calliope/io.py	94.79% <ø> (ø)
src/calliope/preprocess/data_sources.py	100.00% <100.00%> (ø)
src/calliope/preprocess/load.py	92.20% <ø> (-0.20%)	⬇️
src/calliope/preprocess/model_data.py	100.00% <100.00%> (+0.65%)	⬆️
src/calliope/preprocess/time.py	95.74% <ø> (-0.05%)	⬇️
... and 5 more

... and 1 file with indirect coverage changes