Attach swc:subEventOf to the workshop event not to the proceedings

[clange]

As suggested by Olaf Hartig, it's not the proceedings but the workshop event that's a subevent of some conference. Model the RDFa this way (even though it's not straightforward given the existing HTML structure).

[csarven]

See the linked-research branch on how it is approaching this:

<http://ceur-ws.org/Vol-XXX/>
  <http://purl.org/ontology/bibo/presentedAt> <http://ceur-ws.org/Vol-XXX/#event> .
<http://ceur-ws.org/Vol-XXX/#event>
  <http://schema.org/isPartOf> <http://example.org/> .

I think this gives a lot of room to play. Based on the examples, I haven't found a real use for swc.