Add functionality to handle imputed data

[zyteka]

Data imputation can be necessary for training and using ML models, but can lead to interpretability challenges if users don't realize that the values were imputed, and are therefore not real. The simplest, "step 1" way of dealing with this is to have imputations transforms add a flag, marking values that were imputed.

For this issue, a wrapper should be added that will work with common imputation transforms that tracks which items in a DataFrame were imputed.