render natural harmonic / open string (#887)

src/MusicalScore/Graphical/VexFlow/VexFlowConverter.ts

@@ -502,6 +502,10 @@ export class VexFlowConverter {
                     vfArt = new Vex.Flow.Articulation("a+");
                     break;
                 }
+                case ArticulationEnum.naturalharmonic: {
+                    vfArt = new Vex.Flow.Articulation("ah");
+                    break;
+                }
                 case ArticulationEnum.snappizzicato: {
                     vfArt = new Vex.Flow.Articulation("ao");
                     break;

src/MusicalScore/VoiceData/Articulation.ts

@@ -7,7 +7,7 @@ export class Articulation {
 
     constructor(articulationEnum: ArticulationEnum, placement: PlacementEnum) {
         this.articulationEnum = articulationEnum;
-        this.placement = placement;
+        this.placement = placement; // undefined by default, to not restrict placement
     }
 
     public Equals(otherArticulation: Articulation): boolean {