add SRC/REF/SAME property to DESCRIPTOR and EXT elements

[mamund]

NOTE: this is meant to help support a sameAs functionality within ALPS (see #12 )

the SRC (or REF? or SAME?) proprety is an optional property that you can use to reference the source or provenance of a DESCRIPTOR or EXT element. For example, you can define a semantic element with the id property set to "firstName" and a src property set to point to the source defintiion in schema.org (in this case https://schema.org/givenName).

<descriptor id="firstName" type="semantic" src="https://schema.org/givenName" />

[fosrias]

Can src be a url to an ALPS descriptor to show SameAs?

[mamund]

Probably. Would like to explore possible problems but if be happy if that could work.

I'll write up a first pass and we can discuss.

[mamund]

for this release, we'll be adding the def element to descriptors. the value of def will be an IRI that uniquely identifies the defintiion of the descriptor. Note that this might not be the equivalent of sameAs and we can address that separately.