Manage freesurfer license

[AlexVCaron]

Only running the code inside the freesurfer container is not enough, we also need to supply the license file.

We need to find which way is the best to manage this file. It should not be included directly in nf-scil, since it violates the way the license works, but we'll do it (whilst talking with freesurfer) if there is no other good way.