Implement Baart's criteria as a standalone selector object

[dwysocki]

We currently have some hackery in place to deal with the selection of GridSearchCV versus Baart. Instead of implementing Baart's criteria as a selector object, like GridSearchCV, we have it as an option in the Fourier class.

It would be much better if we created a standalone Baart class, which obeyed the same interface as GridSearchCV. This would allow us to simplify the rest of the codebase, and more easily allow for arbitrary selectors, as described in #91.

As another thought, it may be better yet to implement a more general selector object, which takes an arbitrary stopping_criteria function, which could then be set to baarts_criteria or any other function. I wonder if such a thing already exists...