Prevent downloading already downloaded datasets

[quentinduche]

We need this feature to prevent downloading already downloaded datasets.
I think this "filtering" should be performed at the dataset level before downloading datasets.

[alexpron]

I agree it would indeed be needed. At the moment it was not implemented as it represents a subsequent amount of work to be done in a proper way (see https://github.com/datalad/datalad-xnat as an example for xnat).