[feat] expose bfactors in protein_to_pyg function

[kierandidi]

Reference Issues/PRs

What does this implement/fix? Explain your changes

Bfactors are present in df from biopandas, but cannot be saved to pyg object. Now this option is enabled.

What testing did you do to verify the changes in this PR?

Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• [ ] Added appropriate unit test functions in the ./graphein/tests/* directories (if applicable)
• [ ] Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• [ ] Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/protein/test_graphs.py)
• [ ] Checked for style issues by running black . and isort .

[sonarcloud[bot]]

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