Mmmh, I'm not sure: this function helps the user building his own slicing style, rather than manually create it with create_slices.

If we keep it, we could actually merge the current time_generalization and decode_across_time functions, as the latter is identical to the latter, using "diagonal" testing_slices. By merging the two, we could i) make that clearer, and ii) use the same sliding window functions in both cases iii) use the inputs (generalization across conditions etc) and outputs (scores, clf coefficients etc).