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

• 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

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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[jacobcook1995]

Looks really good! I just had a minor LaTeX suggestion

[davidorme]

Looks good to me - some minor issues.