lots of from_string should be classmethod

[njzjz]

Summary

Fix a bug introduced in #3158. A lot of from_string methods should be classmethod.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install

[janosh]

Oops! 😅 Thanks for catching this!