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!
(new) line  147: I split the data (X) in different chunks that I send parallely to the _fit_slices() (line 570).

The main difficulty was related to the possibility of parameterizing the training time of X. I thus had to convert these absolute time values, to time samples relative to each separate chunk: cf lines 597-603.