SRO - renaming

[wshayes]

This would be a major version change, but after doing a survey of the graph/network literature, subject, relation, object is more frequently used than source, relation, target. I'd like to add subject, object as aliases with source, target being deprecated (still available but planned for removal).

[nichtich]

The literature may use different terminology but in other graph formats the terms source and target are also common.

[wshayes]

Not worth the disruption given lack of clear community preference. Closing.