Preprocessor Pipelines can be broken by one-way preprocessors

[eharpste]

Some of the preprocessors are designed to be only one-way (e.g., ObjectVariablizer, NumericToNominal, and NominalToNumeric) this means that their undo functions raise UnImplmentedExceptions. A problem with this is if they are added to a Pipeline they will break the overall undo process. I would argue for changing their undo to just pass through the original object. Alternatively we could put special logic in the Pipeline to catch the exception.

[eharpste]

I'm thinking of handling this by introducing another base class preprocessor of a one-way preprocessor that just no-ops its undo function.

[eharpste]

Fixed previously