Change `diagnosis` into `group` and `subgroup`

[ravih18]

ClinicaDL commands are very Alzheimer's disease oriented as it manipulates labels corresponding to stable AD, stable CN, pMCI, sMCI and non-regressive MCI which are only relevant in AD context. #258

To make it more general an idea would to rename option/column diagnosis into group and subgroup that would later be defined by the user.

The group would be a general label for a session (in the case of AD it would be AD, CN or MCI), and the subgroup can for instance model the stability of the label (stable, progressive, regressive...).