Fix add_sequence_neighbour_vector

[anton-bushuiev]

Reference Issues/PRs

Hi, @a-r-j 👋! Currently, add_sequence_neighbour_vector has two bugs:

• If conditions for adjacency fail, the vec from the previous iteration (pair of adjacent residues) is being used. So, the errors propagate silently. I fixed it to put zeros when conditions fail.
• It does not handle insertion codes. Currently, I fixed it by parsing node ids first, but there probably may be a better way to get insertions.

I have also removed a redundant check for chain identity when iterating through the nodes of the same chain, and added a test.

What does this implement/fix? Explain your changes

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

Pull Request Checklist

• 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]

Thanks @anton-bushuiev, LGTM! Can you please add a short note to the changelog?

[anton-bushuiev]

Hi, @a-r-j ! I have updated the changelog but I am not sure how it is currently with versioning.

[sonarqubecloud]

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