support forming a URI from Coding values

[balhoff]

Currently an instance of Coding looks like this:

collection_site:                                            # type = BodySite
    site:                                                     # type = CodeableConcept
      coding:                                                 # type = Coding
        - code: C12468
          label: Lung
          system: http://ncithesaurus.nci.nih.gov
          tag:
            - harmonized
        - code: Lung
          label: Lung
          system: http://crdc.nci.nih.gov/gdc
          tag:
            - original

While the system value is a URI, this doesn't imply that it can be used as a URI namespace to be prepended to the code, if one wants to derive a URI for the coding value to use in RDF-based tools. In FHIR examples I've seen, this URI is not intended to be a namespace, just an identifier for a system. It would be nice if CRDC-H (or even as a feature of LinkML) we had a defined way to include a URI version of the coding value, or generate a URI from components in the coding instance. If we are going to use the CodeableConcept model, I don't think we need to precisely mirror FHIR. We could support better interoperability with the linked data world.