Support Arkindex datasets for dataset extraction

We'll need to patch the teklia-dan dataset extract command. Obsolete CLI args:

• --XX-folder
• --parent-element-type (we'll simply look for elements under dataset elements)

No more folders, basically you mostly need to rewrite this section of ArkindexExtractor.run

• Instead of iterating over folders, you'll iterate over DatasetElements, per split
• Pass each dataset element to self.process_parent