unconsistent notations

[pbellec]

season 1 is sometimes called s1 and sometimes called s01, and that applies to all seasons.

It would be better to use a consistent notation s01.
This notation is better first, because it's used for the fMRI data and it would be difficult to update and, second, also because it will ensure that alphabetical order matches the order of seasons if ever we acquire s10 one day.