Fix to_pdb for ANISOU records after label_models - #166

Code of Conduct

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Description

label_models() adds an integer model_id column to the ANISOU frame as well as to ATOM and HETATM. to_pdb() only skipped unexpected columns for ATOM and HETATM, so for ANISOU it tried to append the integer column to the output string and failed:

ppdb = PandasPdb().read_pdb("tests/pdb/data/4eiy_anisouchunk.pdb")
ppdb.label_models()
ppdb.to_pdb("out.pdb")
# TypeError: can only concatenate str (not "int") to str

to_pdb() now skips model_id silently, the same way to_pdb_stream() already drops it, and unexpected ANISOU columns are skipped with the same warning used for ATOM/HETATM (checked against the ANISOU record fields).

Two tests added in tests/pdb/test_write_pdb.py: writing the ANISOU test file after label_models() gives back the original file without a model_id warning, and an extra ANISOU column is warned about and skipped. Both fail on main and pass with this change.

Related issues or pull requests

Fixes #124

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable) (the changelog lives at docs/CHANGELOG.md)
• Added appropriate unit test functions in the ./biopandas/*/tests directories (if applicable)
• Modify documentation in the corresponding Jupyter Notebook under biopandas/docs/sources/ (if applicable) (not needed)
• Ran PYTHONPATH='.' pytest ./biopandas -sv and make sure that all unit tests pass (ran pytest tests/pdb: 86 passed, 3 skipped, 7 xfailed)
• Checked for style issues by running flake8 ./biopandas (no new warnings in the changed lines)