Renaming structuredgrid.py to camelCase

[john-science]

What is the change?

Changed the name of structuredgrid.py to structuredGrid.py, so it matches our camelCase naming convention.

Why is the change being made?

This PR was made to close #1608

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify that the new/changed code works.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes (location doc/release/0.X.rst) are up-to-date with any important changes.
• The documentation is still up-to-date in the doc folder.
• If any requirements were affected, mention it in the release notes.
• The dependencies are still up-to-date in pyproject.toml.

[keckler]

Easy. I also checked a bunch of downstream repos and found nothing else that needed to be migrated.