`Stepper.predict` can be made private

[jpdunc23]

As of 49c297c we don't actually use the predict method in any way except internally in Stepper.predict_paired. Therefore, it could be made private, i.e. _predict.

If so, then at the same time it may be worthwhile to rename predict_paired to predict.

[mcgibbon]

In terms of timeline/sequencing, I'm a little worried about making two breaking changes, compared with one smaller breaking change.

If we make predict private first, users will have to update to predict_paired and use the new PairedData type. Then, when we deprecate/remove predict_paired, they will ned to change back to predict.

It feels nicer to our users to update predict to work like predict_paired and deprecate (warn) on calls to predict_paired, and later remove predict_paired. The only change initially would be the predict returning PairedData values instead of BatchData values. If we want to be "backwards compatible" when doing this we can update PredictPaired to temporarily have a .data property method alongside a deprecation warning, which is likely the main attribute that would get used externally (or we could add the others if we're worried about those too).

This is basically your proposal of "at the same time it may be worthwhile to rename predict_paired to predict".