Define licences for the datasets

[thibautjombart]

I would imagine CC-BY or CC0, but this should be checked for each dataset. More info on licences on:
https://creativecommons.org/

@sdwfrost any preferences / strong feelings?

[sdwfrost]

CC-BY or CC0 should be fine. Data are generally not covered by copyright (see here), although the structure of the package should be licensed separately. I prefer MIT, but I'm open to other suggestions.