Add references to the Credit Taxonomy

[marijane]

The roles that map to the Credit Taxonomy need to be annotated as such.
What annotation property do we use for this?

[marijane]

While writing #19, I discovered that this is already done: each role that is mapped to a role in the Credit Taxonomy has an 'obo:definition source' annotation that contains the URL to the appropriate role definition page at the CASRAI dictionary website. Do we want or need to add any further annotations, such as the ID?

I was also reminded that we did not place the Credit roles at the top of our class hierarchy, and I'm wondering if that will be a problem if we want to be able to create subsets of CRO, similar to Uberon ontology subsets...?