Fix to pdb stream

[goniochromatic]

Description

Fixed the bug where PandasPdb.to_pdb_stream() was returning output.seek(0) instead of output, and the default argument for the records parameter in the same function was changed from a mutable to an immutable object, as general best practice.

Fixed some general typos and style issues in pandas_pdb.py, and also updated the link to "How to contribute" in README.md

Related issues or pull requests

issue #108

Pull Request Checklist

• Added a note about the modification or contribution to the ./docs/sources/CHANGELOG.md file (if applicable)
• 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)
• Ran PYTHONPATH='.' pytest ./biopandas -sv and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., PYTHONPATH='.' pytest ./biopandas/classifier/tests/test_stacking_cv_classifier.py -sv)
• Checked for style issues by running flake8 ./biopandas

[a-r-j]

LGTM!

[rasbt]

Thanks!!