We introduce an abstraction layer between the material models and the solver via the MaterialManager class. At the heart of it, the material manager controls (possibly) multiple material models within the microstructure. Phases that are represented with different labels in the microstructure image could belong to different material classes.

This PR also introduces a change in the way material model information is prescribed in the input JSON, which is documented in the README. But I quietly support the old way of prescribing the material information for now to maintain backward compatibility.

This would also address and hopefully close #49.

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.