VBM on Oasis cross-sectional dataset (> 400 subjects)

Added a downloader + an example for the Oasis dataset that we processed with Elvis. We put the archive containing the data on NITRC.

DARTEL version will come later.

I had hard time setting up the downloader: sometimes, it downloaded the same archive twice (I do not know why). Everything seems to be ok now but it would be cool if someone can double-check.
Thank you.