Unit handling and round-trip .nc conversion fixes

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No description provided.

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Closes #10

[hassec]

Before merging this I'd like to make sure we have a way to reproduce the error described in #10. I've tried locally without luck so far and added a test that would catch the in #10 described problem, see #14

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Think I've figured out how to reproduce the error. To fix https://github.com/cfs-energy/cfspopcon/actions/runs/7038938605/job/19156931593?pr=7, I added netcdf4 to the poetry environment. Then I can reproduce #10 with

poetry add netcdf4
rm -rf .venv
poetry install
poetry run python tests/regression_results/generate_regression_results.py
poetry run pytest

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I broke it https://github.com/cfs-energy/cfspopcon/actions/runs/7039353397/job/19158160776?pr=13 🥳

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And then I fixed it again https://github.com/cfs-energy/cfspopcon/actions/runs/7039432837/job/19158407447?pr=13 🥳🥳🥳

[hassec]

Just a small change request to add a comment to the conversion logic.

I'd prefer to have dependency updates to the poetry.lock in a separate PR but since you already fixed the mypy stuff here let's merge this as is. But in the future we should try and avoid poetry updates in PRs that have other changes.