Make data release folder visible to XNAT

Data should be visible under /data/xnat/archive/$ProjectID/arc001 so that they can be associated to MRSessions

However, this solution breaks XNAT's validation when new sessions are imported from REST. On top of that, XNAT byproducts (XMLs, snapshots, ...) would pollute the original dataset.

At the moment, the best solution seems to be:

• create the actual folder hierarchy matching XNAT's expectation as to data organisation (arc0001/SESSION/...)
• generate one symlink per target file
• register the symlink in the REST call

This would link to the pipeline generating the XML to be imported via REST by passing the (base)path to the newly created symlink as a parameter to the converter.

TODO

• see whether we can register valid but not existing path and then generate the symlink with a pipeline