Retain more APs in imports

[pbuttigieg]

Unlike other ontologies, we may want to retain some more APs on SDGIO imports (e.g. definitions, comments, elucidations) to make sure they're visible on UN Environment landing pages. Cross-ref #26

[Mttbnchtt]

This is a good idea. Do you have in mind who are the plausible users of SDGIO? This may help to create APs with explanation, examples, and possibly references to relevant material.