feat(Publication): [AoU-493] add number of files property

https://issues.unige.ch/browse/AOU-493

AoU-Model/src/main/java/ch/unige/aou/model/publication/Publication.java

@@ -303,6 +303,10 @@ public class Publication extends AouResourceNormalized<Publication> implements P
     return commentOpt.orElse(null);
   }
 
+  public int getNumberOfFiles() {
+    return this.documentFiles.size();
+  }
+
   @JsonIgnore
   public PublicationStatus getDefaultStatus() {
     return PublicationStatus.IN_PROGRESS;