[osis-core] Link Attributes

[Harry Plantinga]

> I think you need
>   (a) a pointer to another document that this "comments on" or "annotes"
>   (b) an attribute in which you can put in a type (string).
> 
> Is the difference in (a) and (b) that (a) is a statement about the role 
> of the resource and (b) is a statement about the type of resource? 
> 
> I like your referenceType, perhaps extend it to referenceRole with some 
> typology of "comments on", "annotates" ?

The purpose of (a) I had in mind is is to identify the document (or more 
abstract pointer, e.g. Matt.1.3).

The purpose of (b) is to say what kind of annotation relation it is, e.g
"isTranslationOf".

At first I had thought to overload outCite for (a), but something could
also be said for a separate attribute, "annotates".  Either way, (b) would
be a way of saying more about what type of annotation relation it is.

If we were to enumerate a number of standard relationships, I would want
to be able to extend the list.  E.g.

"commentary"
"response"
"translation"
"exposition"
"poeticRendering"
"x-MyCommentsOn"
"x-MarginalScribbles"

You know, this could be really useful.  You could write up notes on 
some document, respond, hold a whole scholarly conversation this way.

-Harry