add provenance metadata to XMIs

[mromanello]

(jotting down some quick notes, will rephrase later)

• [x] we need to create a DocumentAnnotation layer in the inception project
• [x] the DocumentAnnotation layer is experimental and needs to be enabled in inception (settings/properties file)
• [x] set the metadata values in the script which produces the XMIs

[mromanello]

For now I have configured in inception the following provenance related metadata fields:

• ocr_run_id
• regions_considered (comma-separated string)
• xmi_creation_date

@sven-nm Can you think of anything else that we may need to store there and retrieve after annotation?