review cross-ontology-diff datamodel prior to implementing

Currently the cross-ontology-diff code uses an ad-hoc tuple as the output:

https://github.com/INCATools/ontology-access-kit/blob/52771fef55cce0749863c3ea45a55bd7110f785e/src/oaklib/omk/omk_mapping_utils.py#L14

current results look like this:

```
('IDENTICAL', 'EMAPA:16037', 'rdfs:subClassOf', 'EMAPA:36032', [])
('OTHER', 'EMAPA:16041', 'rdfs:subClassOf', 'EMAPA:16039', [])
```

which is not so useful for review. We want to see labels, plus mapped concepts

We would like to move to a documented datamodel that may have applications beyond this software library:

https://incatools.github.io/ontology-access-kit/datamodels/cross-ontology-diff/index.html

Review this prior to implementing