Add an input arg check for Kpoints.automatic_density_by_lengths

[Andrew-S-Rosen]

Summary

The function Kpoints.automatic_density_by_length() takes in a sequence of values, e.g. [50, 50, 1], to set the k-point density for each dimension. However, the function does not raise any kind of error if a sequence of dimensions != 3 is supplied.

This PR adds a ValueError if the len is != 3, along with a test for it.

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]

Thanks @arosen93!