NIHVIVO-3878 update RDFService to handle N-TRIPLE format serialization

webapp/src/edu/cornell/mannlib/vitro/webapp/rdfservice/RDFService.java

@@ -17,7 +17,7 @@ public interface RDFService {
 	}
 
 	public enum ModelSerializationFormat {
-	    RDFXML, N3
+	    RDFXML, N3, NTRIPLE
 	}
 	
 	public enum ResultFormat {

webapp/src/edu/cornell/mannlib/vitro/webapp/rdfservice/impl/RDFServiceImpl.java

@@ -143,6 +143,8 @@ public abstract class RDFServiceImpl implements RDFService {
                 return "RDF/XML";
             case N3: 
                 return "TTL";
+            case NTRIPLE: 
+                return "N-TRIPLE";    
             default: 
                 log.error("unexpected format in getFormatString");
                 return null;