[FEA] Update statistics of an existing workflow

[EvenOldridge]

In addition to workflow.fit() and workflow.transform() we need a workflow.update() which takes the existing fit and updates the statistics.

For most ops this should be very straightforward, but may require us to capture additional data associated with the statistics like the number of entries.

This feature is a key part of continuous training of recommender systems.

[gabrielspmoreira]

Issue #798 addresses the incremental pre-processing of categorical features.