Remove Need for ConceptSet Manipulation from API

Currently ConceptSet is treated and exposed like any other domain object. We want to hide this behind the API.

Add a method to the Concept class to get direct (immediate) members of a set like:

if (concept.isSet())
  List<Concept> concept.getMembers();

and add a method to ConceptService to get all members (turning any child sets into their members):

if (concept.isSet))
  List<Concept> Context.getConceptService()
    .getAllConceptMembers(concept);

Nowhere does the API consumer need to know or be concerned about ConceptSet. ConceptSet disappears from the API (domain objects and service methods). Whether or not concept_set_derived is used becomes an internal performance issue hidden within the API.

Decided on this dev call.