skos predicates vs xref

[cmungall]

We have both skos-based mapping predicates

• https://w3id.org/biolink/vocab/exact_match
• https://w3id.org/biolink/vocab/narrow_match

and

• https://w3id.org/biolink/vocab/xref

We need guidance as to when to use which. I recommend even deprecating xref and forcing people to choose a more precise predicate

(note an orthogonal difference is the range, with the match predicates having a range of namedthing and xref being an iritype - but we can use edges as node properties so this shouldn't be an issue)

See also #210

[sierra-moxon]

@kevinschaper - on ingest, do you have enough data to determine the match quality of most xrefs?