basic toString method implemented

model/src/main/java/lcsb/mapviewer/model/map/reaction/Reaction.java

@@ -925,4 +925,9 @@ public class Reaction implements BioEntity {
   public double getSize() {
     return 0;
   }
+  
+  @Override
+  public String toString() {
+    return new ElementUtils().getElementTag(this);
+  }
 }