Document "Replace" Transformation

[adelavega]

@rotemb9 suggested this while trying to create NARPS model

"2. The model I was working on has a quite common feature: it uses a different duration for the regressors than the duration of the trial given in the tsv file (specifically, it uses the average TR). Jeanette told me it can be done by assigning the desired duration to the variables, but we should definitely show examples of that for users because this could be confusing."

[Remi-Gau]

I honestly think that the transformations should have their documentation (in the jupyter book or wherever they are hosted) with an example of what each of them does.

No matter how much we'd like to, we won't be able to use transformation.

What would be ideal would be a website / binder where on an example pandas dataframe one can "chain" transforms one after another to see what they do.