490 hydrology docs on implementation

[vgro]

This PR adds the implementation of the hydrology model and updates the variables description in the abiotic simple model to match the plants. There is a problem with loading the elevation data from netcdf, I'll try and figure that out soon.

https://virtual-ecosystem.readthedocs.io/en/490-hydrology-docs-on-implementation/virtual_ecosystem/implementation/hydrology_implementation.html

Fixes #490

Type of change

• [x] 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

• [x] 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
• [x] Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.93%. Comparing base (fb2fea3) to head (6eb3ecd).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #518 +/- ## ======================================== Coverage 94.93% 94.93% ======================================== Files 73 73 Lines 4048 4048 ======================================== Hits 3843 3843 Misses 205 205 ```

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