add "transform" optional step to pipeline

[PeterDSteinberg]

Currently in the config's pipeline section (a list of dictionaries indicating training or prediction actions), the actions must have the keys:

• train, or
• predict

We should add a 3rd one for transform to cover the cases of models that do not have a predict method but rather a transform and/or fit_transform. Included in this work would be ensuring the transform actions:

• can be specified in config, perhaps in the sample_pipeline section of a pipeline step
• can serialize / deserialize the outputs of transform for transforms like PCA

[PeterDSteinberg]

Fixed by #45