Refactor code quality

[cdgaete]

Changes proposed in this Pull Request

• Add new .github/workflows/type-checking.yml workflow to run mypy static type checking on the codebase
• Fix type errors in core.py by properly casting Path objects to strings (join function requires string paths)
• Update dependencies in pyproject.toml:
  • Add mypy and required type stubs to dev dependencies (types-requests, types-PyYAML, pandas-stubs, types-tqdm, types-six)
  • Add a new 'plotting' dependency group for optional cartopy dependency
• Add mypy configuration in pyproject.toml with appropriate settings for our code
• Silence mypy "misc" errors in the accessor.py file
• Configure external library import ignores for third-party packages

Additional improvements:

• Update pre-commit config with end-of-file-fixer, trailing-whitespace and check-merge-conflict hooks
• Fix cycler import in plot.py to avoid matplotlib deprecation warning
• Fix various whitespace and end-of-file issues throughout the codebase (automated)

@euronion @FabianHofmann

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[euronion]

This looks very nice. Let us know when it is ready for review.

[cdgaete]

Hey @lkstrp, I'm submitting this PR focused on code quality improvements, which should help streamline our ongoing development work. There will be additional PRs coming in the next few weeks, but I wanted to prioritize this one first. As we discussed previously, I've postponed the doc-string enhancements for a later phase. Would appreciate your review when you have the opportunity!

[lkstrp]

That's perfect, thanks a lot @cgaete!
I just opened the other PR to get the pre commit diff out here. Will review and merge soon

[lkstrp]

No need to change for this PR, but just the note that we wanna move to pathlib in general. Not sure if mypy would be happy without the str cast then