Soil moisture units to mm in data object

[vgro]

This PR changes the soil moisture unit to mm in the data object. For some calculations in the hydrology and abiotic model, it is still needed in volumetric moisture content, but I think I reduced the number of conversions. I also tidied the code based on what we have learned, but there is still a big outstanding issue with accessing indices etc.

@jacobcook1995 could you add the changes needed in the soil model to this branch please?

Fixes #345

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [x] 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

[codecov-commenter]

Codecov Report

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

Project coverage is 94.52%. Comparing base (13ef532) to head (faf26e6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #420 +/- ## =========================================== + Coverage 94.50% 94.52% +0.02% =========================================== Files 68 69 +1 Lines 3583 3598 +15 =========================================== + Hits 3386 3401 +15 Misses 197 197 ```

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