506 implementation of abiotic simple model docs

[vgro]

This PR describes the implementation of the abiotic_simple model.

This has gone through a lot of iterations as we are not sure how we want to split information between theory and implementation, i.e. what is actually useful for the users. Any comments/ideas are welcome.

https://virtual-ecosystem.readthedocs.io/en/506-implementation-of-abiotic-simple-model-docs/virtual_ecosystem/implementation/abiotic_simple_implementation.html

Fixes #506

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 94.91%. Comparing base (0f840e9) to head (8f8f5af).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #515 +/- ## ======================================== Coverage 94.91% 94.91% ======================================== Files 70 70 Lines 3857 3857 ======================================== Hits 3661 3661 Misses 196 196 ```

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