Update addons.md

[AndySAnker]

Added DebyeCalculator as an external tool

Summary

Major changes:

• feature 1: ...
• fix 1: ...

Todos

If this is work in progress, what else needs to be done?

• feature 2: ...
• fix 2:

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install