Add representative image to Dataset

[agbeltran]

In order to display it in Graphical User Interfaces, it would be useful to have a representative image associated with a Dataset.

I suggest we add an image field to the Dataset class (https://repo.icatproject.org/site/icat/server/4.10.0/schema.html#Dataset). The ingestion process would select what that image should contain.

[RKrahl]

Just an idea, not sure if it is really useful: if we worry about the Dataset table to become too big because of all those thumbnail images, we could also store the thumbnails as files in the storage and make that image attribute in Dataset a relation to Datafile.