Pandas deprecation fixes

[koen-vg]

Just fixing some deprecation and future warnings as I go along :)

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[koen-vg]

Ah, this time the tests saved me; I was a little too eager with just one of the fixes.

[fneum]

Looks good! Thanks! Just unsure about two cases that I couldn't verify without testing it myself.

[koen-vg]

Yes you were right! At least the one I was able to test, in the weighted price calculation. It turned out the code path wasn't used before due to an unrelated bug, which I now also fixed. I will say that the whole function seems pretty fragile...

For the CO2 emissions calculations, the code path you pointed out is also never taken! Hence why I didn't find the mistake. It turns out that in sector-coupled perfect foresight networks there aren't any co2-emitting generators anyway; all emissions go the the co2 atmosphere store.

[fneum]

Great! Thanks for looking into it and sorry that the script is so buggy...

[fneum]

test failures seem unrelated, I will merge given previous commit CI ran through.

[koen-vg]

No worries, happy to help and anyway it's so much better to have anything at all to get started with than having to do everything scratch!