Round-tripping with non-CSV originating models

[mattmcgrattan]

Currently, models that are initiated solely in Omeka S, exported as JSON, and converted to CSV using gen_csv.py will not export back to JSON using gen_json.py cleanly.

This is because the Capture Model importer/exporter makes use of an ID field, and sets the Type (neither of which are native to the Crowd Source Element and Group resource templates).

Add better handling for missing IDs to the gen_csv/gen_json code, and, consider persisting any existing IDs from JSON as the ID in the exported CSV.