Add 'describes' provenance information

[martinjoconnor]

From #695:

"In the model currently in use, CEDAR describes metadata about the instance, not about the original object itself. (At the element level, it is describing metadata about the element, not about any lower-level object itself.) This is useful in many CEDAR contexts where much of the metadata is not actually about the original entity, but about (for example) the person filling out the form or submitting the metadata."

We could potentially add a provenance field to cedar instances to indicate the actual entity being described.

Marcos suggested the Semantic Science Integrated Ontology (SIO) describes property:

http://semanticscience.org/resource/SIO_000563.rdf

[martinjoconnor]

A related field would be the public identifier of the original resource, something CEDAR does not currently have (#786). This would not be a URI but would instead be a string to hold source-provided identifier.

[martinjoconnor]

Also related to #780.