Release v0.9.1

chore(release): phd-thesis 0.9.1

This release is about fixing up the machine learning chapter. There are
significant improvements to the introduction and the figures.

planning: Restructure ML chapter.
editing(ML): Fixing up and clarifying introduction.
projects: Update projects.
projects: New figures from MLCrystals project.
editing(ML): Improve description of learning methods.
writing(ML): Incorporate goals of ML chapter.
editing(ML): Clarify results.
planning(ML): Include conclusion for ML chapter.
figures(ML): Fix figures in ML chapter.
projects: Update figures in ML and Melting.
planning(methods): Move more ML methods.