Refactor Amset class into separate subclasses

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Currently, the Amset class is very monolithic, with the core.py file running to almost 4,000 lines of code. To improve maintainability, I think it would be good to refactor out bits of the class into separate subclasses.

Initially, I think the plotting and io functions are ideal candidates for refactoring. We could do what pymatgen and other packages do, and just have an AmsetPlotter class that takes the Amset class as an initialization parameter.