This describes is the format in which the data is internally stored, and what `self.get_data()` will return.
I think it is mainly used in the code doing the preprocessing, because it accesses the internal data directly.
I'm not convinced it is the right way to do it, maybe we should make it private, or give it another name, but it was convenient to have it, at least for the moment.
I'm adding this in docstrings.