kitodo
Make THUMBS fileGrp optional when using image servers
Description
When using image servers like IIIF, IIP or Zoomify the file group THUMBS should be optional, since those derivates could be fetched from the image server as well.
Problem
Even when using an image server it's still mandatory to have an additional file group THUMBS. This is redundant and just makes the METS files larger.
Proposed Solution
If the file group DEFAULT refers to an image server, this should also be used for fetching thumbnails.
Target Version
This doesn't break backwards compatibility and could be implemented in any future version. But it should go with an update of the application profile to reflect the change.
Additional Context
This feature request was submitted via DFG-Viewer mailing list: https://maillist.slub-dresden.de/pipermail/dv-technik/2021-February/001079.html
