Feature/add streamflow

[vgro]

This PR adds a super simple stream flow equation to the hydrology model: Q=P-ET-dS . This requires the plant model to run first, but as the p-model does currently not need soil moisture as an input, that should work. I added ET to the dummy plant data set and made a small adjustment to the input precipitation. Once we add p-hydro or SPLASH, we will have to rejig this; maybe split the stream flow from the hydrology model.
For now, the new features gives us the stream flow Q for each grid cell which can then in a next step be summed up for the whole area or catchments within the study area.

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]

LGTM!