519 review abiotic simple and hydrology docstrings to match updated documentation

[vgro]

This PR updates the docstrings for the abiotic_simple and hydrology models. Nothing major, only a few bits that were outdated or inconsistent.

Also, I (was trying to) get rid of the variables that are initialized but not needed in the hydrology model. This led to an old problem of random model order on Windows which caused some tests to fail. The change needs to be postponed until the variables system is up and running, see #522

Fixes #519

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
• [x] All tests pass: $ poetry run pytest

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] 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 (bfe28c2) to head (3c49c6c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #521 +/- ## =========================================== - Coverage 94.93% 94.93% -0.01% =========================================== Files 73 73 Lines 4050 4048 -2 =========================================== - Hits 3845 3843 -2 Misses 205 205 ```

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