More minor code for nodegroup elements

[nilsdeppe]

Proposed changes

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Code review checklist

• [ ] The code is documented and the documentation renders correctly. Run make doc to generate the documentation locally into BUILD_DIR/docs/html. Then open index.html.
• [ ] The code follows the stylistic and code quality guidelines listed in the code review guide.
• [ ] The PR lists upgrade instructions and is labeled bugfix or new feature if appropriate.

Further comments

[nilsdeppe]

I've rebased and pushed fixups. I added a decent number of words for the return type being size_t since that is important. I'll need to clean this class up a bit after everything is working, mostly just the documentation. I also added value_type as a type alias since this is actually what the STL uses. It's not used right now, though.

[nilsdeppe]

Done! Thanks!