Support cross_val_predict for stacked estimators

[skylarbpayne]

First -- this library looks great, pretty much exactly what I was looking for!

It appears though, that each dependent estimator is trained on the entire passed dataset directly. This can lead to overfitting. Instead, it would be nice if cross_val_predict was supported in some fashion so when we have estimators A -> B, B is trained on output of cross_val_predict rather than raw predict output.

Given activity in this repo, I'm assuming its basically inactive. But if not, I'd be happy to contribute such functionality.

[big-o]

Hi and thanks for the feedback! The project is very much active, although I'm running it alone in my spare time so may be sporadic and slow to reply - apologies for that.

I would have expected the sklearn cross_val* functions to work since the DAG should respect the estimator API, so if it doesn't then this sounds like a bug.

If you're interested in contributing a PR that would be very welcome and I'd be happy to review it.