Add utility function to retreive cv preds from a model

Description

Since the format of the cross-validated preds is somewhat non-standard in H2O, I propose we add a utility function that will convert the cv preds into a single column.

Similar to the `pred` object below (maybe we should automatically return a second column with the fold ids too):

Assignee

Erin LeDell

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Reporter

Erin LeDell

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