Tweaks to the Archives proposal

Following an active and interesting discussion at the LODLAM Summit 2017  (notes from the session here) I have made some small tweaks to the proposed model.

• Added a new property holdingArchive to the ArchiveCollection Type
  This is an inverse property to the archiveHeld property of Archive.  Although the Schema.org community does not encourage inverse properties I believe we may have good pragmatic case here where the use-cases could include an Archive holding a significant number of collections which would preclude their individual listing on the Archive type.
• Confirmed that itemLocation is a property of ArchiveItem and replaces the originally proposed use of location.

I have updated the model diagram and examples on the proposal page in the wiki to reflect these changes.  They have also been reflected in the Archive prototype Schema.org instance.

If people are happy with these changes, I suggest we move to proposing this initial extension to the broader Schema.org community.

I have had some people indicate that if it was accepted they would be prepared to look at implementing it in systems to which they have access/influence.   Such support always helps with the submission of proposals to Schema.org, please let me know if a) You could make such a commitment and b) if you would want that to be known publicly or not.