exposed fill_value to protein_to_pyg function

[kierandidi]

Reference Issues/PRs

What does this implement/fix? Explain your changes

Exposing the fill_value for atom37 representations to the protein_to_pyg function so that the user can choose this and it is transparent where the value comes from.

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]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 45.05%. Comparing base (8123f42) to head (2458b68). Report is 169 commits behind head on master.

:exclamation: Current head 2458b68 differs from pull request most recent head 739cafb. Consider uploading reports for the commit 739cafb to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #385 +/- ## ========================================== + Coverage 40.27% 45.05% +4.78% ========================================== Files 48 113 +65 Lines 2811 7919 +5108 ========================================== + Hits 1132 3568 +2436 - Misses 1679 4351 +2672 ```

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