update reference model datatype (include identifier model)

frontend/src/lib/models/datatypes/identifier-model.ts

@@ -0,0 +1,8 @@
+import {CodableConceptModel} from './codable-concept-model';
+
+export interface IdentifierModel {
+  use: string
+  type?: CodableConceptModel
+  system?: string
+  value?: string
+}

frontend/src/lib/models/datatypes/reference-model.ts

@@ -1,4 +1,7 @@
+import {IdentifierModel} from './identifier-model';
+
 export interface ReferenceModel {
   reference: string
   display?: string
+  identifier?: IdentifierModel
 }