Access whitening transform models

There is no way for the user to access the models produced by the whitening transform (the whitening matrices). We would need to expose that, as well as a function that allows to apply the matrix to new suitable input vectors (something similar to FillValues in the current code).

Furthermore, we need to have an onFit function for pigsty to pass this class out.

Adding @Ivanidzo4ka to the conversation.