Feature/update hydro model

[vgro]

After discussion the limitations of soil parameterizations, I decided to remove the vertical flow from the soil moisture update and only keep the function for a 'mean flow rate' with lots of warnings in the comments (potentially too many repetitions). We can find a solution later together with the soil and plant model.
I also simplified the code a little bit by calculating intermediate variables that are used several times at the start, and I corrected how available capacity is calculated (sum of layers rather than mean)

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

[jacobcook1995]

Looks sensible to me!