Remove duecredit

[tsalo]

Summary

Duecredit is currently an optional dependency, and I don't think many people use it. I also think that the docstrings' BibTeX references and plans for improved workflow boilerplates could be good enough for most users.

@jdkent have you thought about how the NIMADS CLI might generate boilerplates? Or will it not?

Additional details

Fewer dependencies to think about, and less work for devs when adding a tool or method.

Next steps

1. Remove the duecredit dependency group.
2. Remove the duecredit stub file.
3. Remove uses of duecredit from the codebase.
4. Remove the references.py file.