ENH: Add support for cross generalization for time_gen

@dengemann @agramfort

This is work in progress, I will let you know when to review it.

Thanks!
if you parallelize across samples you can use a few jobs too if
each job takes care of an interval and not a single sample.

I pretty convinced this option is more pain than good (more code to maintain, more tests to run, more options to explain/document, ...)