Introduce Myxboard concept

[salma-remyx]

Allow users to create a place where they can evaluate multiple facets of an experiment (models, evaluations, dataset designs, etc), what we're calling a "Myxboard".
This comes with a restructuring of the api.py and cli.py files to allow for a pattern where the API is instantiated and acts upon Myxboard objects and other data objects to come in the future.
See Design Doc