Support Broadcasting the FHIR ValueSet

[bdrillard]

A FHIR ValueSet is described as

... a set of codes drawn from one or more code systems.

It would be useful to be able to read and broadcast FHIR ValueSets in a manner similar to how BroadcastableValueSets supports LOINC and SOMED codes. FHIR ValueSets are perhaps a simplified form of the relationship-structured LOINC and SNOMED, as FHIR ValueSets have no notion of an ancestor or descendant.

At the moment, our BroadcastableValueSets uses the ConceptMaps class underneath, where the Source and Target systems are the same. I think there's some room for simplification as part of this issue, in which we create a ValueSets class that can capture collections with relationships between their codes (e.g. LOINC, SNOMED), and collections that are flag (e.g. a FHIR ValueSet). ConceptMaps can be disentangled from that implementation and left to be exposed as BroadcastableConceptMaps for mappings between distinct Source and Target systems.

[bdrillard]

This issue was resolved with #5, but a final note here that rather than use of FHIR ValueSets for hierarchical systems like LOINC and SNOMED, #5 adds the classes Hierarchies and Ancestor which makes up the underlying support for broadcasting transitive codes.