Add unit tests for the fitting math in fit.py - #70

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938, actions/setup-python@f677139bbe7f9c59b41e40162b753c062f5d49a3. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/