Create a git-annex repo for data from `basel/6900_nifti_20`

[plbenveniste]

The data is stored in duke/mri/basel/6900_nifti_20 and the segmentations in duke/mri/basel/6900_20_lesions-PD The steps are the following:

• [ ] A git-annex repo should be created :
• [ ] The data should be Bidsfied
• [ ] The data should be pushed to the repo
• [ ] A PR should be opened
• [ ] The PR should be merged.

@jcohenadad What name should be used for the git-annex repo ? ms-basel-pd-6900 ?

Another strategy could be to merge this data with the other dataset under construction from basel (issue #19)