Expand the context of use for the value set reference extension to support use on the Codeable Concept

The example is for QI-Core capability to reference a value set extension to indicate that a a code (I.e., the thing not done in this ProcedureNotDone profile) to reference a single code (one thing not done), more than one code (several things not done) and a uri (nothing in the the entire value set was done - I.e., "I didn't do any of these"). To allow more than one thing not done, the value set reference would need to be a Codeable Concept for the reference extensions. (see: http://hl7.org/fhir/us/qicore/StructureDefinition-qicore-procedurenotdone.html). There are 8 QI-Core "notDone" profiles that would benefit from this change

1. QICoreCommunicationNotDone
2. QICoreDeviceNotRequested
3. QICoreImmunizationNotDone
4. QICoreMedicationAdministrationNotDone
5. QICoreMedicationDispenseNotDone
6. QICoreMedicationNotRequested
7. QICoreObservationNotDone
8. QICoreProcedureNotDone
9. QICoreServiceNotRequested