Change mapping_date to be dcterms:created

[nichtich]

The use of pav:authoredOn only makes sense if pav:createdOn is used as well to differentiate two types dates, in addition to the publication date. SSSOM only has one type of date so there is no need to not use plain old dcterms:created.

Requires #456.

[gouttegd]

@matentzn Any reason not to do this? If we want to do it (no objection on my part), now would be the time.

[matentzn]

I am fine with making that change.

[gouttegd]

Good. I’ll add it to the RDF spec PR, since the change is only really meaningful for RDF (well, and for JSON-LD but since the JSON-LD spec does not exist yet… :p ).