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!
Why don't I see an isolated commit somewhere. For the future avoid huge commits that change a lot. After some time and for spectators it's super difficult to catch up the essence of it  if the diff is just huge.  I'll need to dig into that in a free minute.