Number of rows in `AFQDataset.X` depends on `concat_subject_session`

[arokem]

I think that this is somewhat related to #132, but a separate issue. When reading a dataset with multiple sessions per subject using AFQDataset.from_files, the number of rows in AFQDataset depends on the value of the concat_subject_session key-word argument. The correct value of subject/session combinations appears only in the case where concat_subject_sesion = True. Otherwise, some rows (subject/session combinations) that should have appeared in the result do not appear (less rows than expected).