Implement ConfigureableMixin in the main branch and refactor

[crasanders]

The ax branch implements a ConfigureableMixin class: https://github.com/facebookresearch/aepsych/blob/ax-backend/aepsych/config.py#L360

This class allows any other AEPsych classes that implement from_config to fetch config arguments in an object-oriented way, allowing code re-use between super- and sub-classes. Implementing this in the main branch. See this PR for an example: #376