Unit handling and round-trip .nc conversion fixes

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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 🥳🥳🥳