Reconfigure initial plant cohort data

[davidorme]

Description

As noted in #1041, the current path for loading initial plant cohort data shoves it into the Data object, from where it is read once and never touched again. That also adds an arbitrary single use cohort_id axis into the Data object.

This PR swaps that data out into a separately configured CSV file of initial cohort data, which is both easier for users and avoids complicating the Data object structure.

• Removes the plant_cohorts_pft, ..._dbh, ..._n, ..._cell_id variables from required __init__ vars in the Data object.
• Instead, these data are provided via the new PlantsModel(..., cohort_data: pd.Dataframe) argument.
• The model schema is updated to add the cohort_data_path config option and the PlantsModel.from_config() now loads that configured file and passes the resulting data frame in to PlantsModel.
• The PlantCommunities.__init__ now takes that dataframe as an input rather than the Data object.
• All tests are updated
• The example_data directory is also updated to the new configuration.
• Updates the example TOML file in docs/source/_static, which was last updated in the Stone Age.

Fixes #1041

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

❌ Patch coverage is 59.45946% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.56%. Comparing base (10d4bc4) to head (537d263).

Files with missing lines	Patch %	Lines
virtual_ecosystem/models/plants/plants_model.py	50.00%	11 Missing ⚠️
...mple_data/generation_scripts/plant_example_data.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1066      +/-   ##
===========================================
- Coverage    94.67%   94.56%   -0.11%     
===========================================
  Files           79       79              
  Lines         6964     6973       +9     
===========================================
+ Hits          6593     6594       +1     
- Misses         371      379       +8     

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