add additional hetatm info to pyg parser

Imposes a better data structure on hetatm metadata with additional fields when using protein_to_pyg.

[ ] 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 .

a-r-j / graphein