Merge `Estimator.summary()` and `Estimator.fit()`

[tsalo]

In PyMARE, fit and fit_dataset both return Estimators, rather than Results. Instead, there's an additional step of calling Estimator.summary() to produce the Results object. This is different from NiMARE, where fitting an Estimator returns a MetaResult.

I don't know if the extra step is necessary. I.e., does fit returning an Estimator instead of a Result serve a purpose?

[tsalo]

Actually, for the sake of scikit-learn styling, what if we instead merged fit and fit_dataset, and then had fit (returns fitted Estimator), fit_transform (returns Results), and transform (returns Results)?