Publish the predicted elements to Arkindex

Depends #3

Instead of printing to stdout, we will now use the API to publish the elements to Arkindex.

• Regroup the predictions by type
• Add a new key name on each dict with value str(index), index being the index in each grouped list
• Reformat (if needed) the polygon to have a list of coordinates and coords[0] == coords[1] (Arkindex compatible)
• Call create_elements with this payload.