Run validator before running ndmg

This way if user tries to run the app on a dataset that is not BIDS (or not quite BIDS) they will get a meaningful error.

To install the validator:
https://github.com/BIDS-Apps/freesurfer/blob/master/Dockerfile#L28

To run it:
https://github.com/BIDS-Apps/freesurfer/blob/master/run.py#L49
