#334 Allow for hook for retrieving further Xena files on export

Export (13)
Terry O'Neill

Currently when an export or such other function (normalisation, etc.) is performed on a Xena file that has links to further Xena files (e.g. a normalised zip file; which results in multiple Xena files) then it is expected that these files reside in a defined file system location relative to the normalised file. It would be useful if the programming of Xena allowed for linking into this functionality to provide for a Xena file that needs to be retrieved in a different way.

The impetus for this change is a change to the Digital Preservation Recorder that will result in Xena files for AIPs being retrieved from the server using a DPR specific protocol before being available locally. This change would then allow DPR to retrieve further files as needed.