Results objects break if user called fit() rather than fit_dataset()

[tyarkoni]

The various results classes all currently require access to the estimator's last-fitted Dataset, which won't exist now following the API change to allow fit() to be called with numpy arrays. The solution is to either implicitly create and store a Dataset, which would be easier, but feels counterproductive (since the point of eliminating Dataset inputs from fit() was to cut down overhead), or to directly store references to the input arrays rather than to a Dataset in the estimator (in which case we have to add a bunch of code to every estimator, and lose some of the benefits of the `Dataset abstraction).

[tsalo]

Based on local testing with NiMARE, I think this is specific to the CombinationTest classes.

EDIT: Sorry, I jumped the gun there. The issue was that the CombinationTest classes behaved confusingly with Datasets, so I used fit() for them and fit_dataset() for the other Estimators. Just ignore my comment...