enable pretty print for disag calc results

src/main/java/gov/usgs/earthquake/nshmp/DisaggCalc.java

@@ -64,6 +64,7 @@ public class DisaggCalc {
 
   private static final Gson GSON = new GsonBuilder()
       .serializeSpecialFloatingPointValues()
+      .setPrettyPrinting()
       .serializeNulls()
       .create();