Make is_valid_poscar less noisy

[Andrew-S-Rosen]

Summary

Closes #421. Since is_valid_poscar will return False very frequently, we should not have warning-level logging messages sent to the user in such instances.

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

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.66%. Comparing base (20cefaf) to head (ba22dc8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #422      +/-   ##
==========================================
- Coverage   53.73%   53.66%   -0.07%     
==========================================
  Files          39       39              
  Lines        3562     3557       -5     
==========================================
- Hits         1914     1909       -5     
  Misses       1648     1648              

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