Open question to all the developers: should we be using a pandas dataframe here?

I think that in the long run, we should be using more and more pandas datasframes in the example code and for behavioral variables. Indeed, when they are well used, they make the code more readable. Thus to foster adoption this could be something important.

Right now, pandas is a package that has enough adoption and install basis that I feel it is safe to depend on it. Nonetheless we could delay the import to inside the function, so that if pandas is not installed, the whole import of nilearn does not fail.