Merge pull request #205 from PhasesResearchLab/refactor/debye-gruneisen

This PR refactors the Debye-Grüneisen implementation and its tests, introducing a new plot_vt plotting API and adding a tutorial notebook demonstrating the DebyeGruneisen workflow.

Changes:
- Refactor DebyeGruneisen to use static calculation helpers and add plot_vt() for plotting vs temperature/volume.
- Update tests/test_debye.py to align with the refactor (new plotting API + direct tests for static helpers).
- Add a new example notebook for running DebyeGruneisen in Codespaces and plotting results.

Author: nhew1994