Added the possibility to get known keys

CometeSwing/src/main/java/fr/soleil/comete/swing/chart/data/DataViewList.java

@@ -61,6 +61,15 @@ public class DataViewList implements ChartObservable {
         return viewMap.size();
     }
 
+    /**
+     * Returns the known DataView ids, as a {@link Collection}.
+     * 
+     * @return A {@link Collection}.
+     */
+    public Collection<String> getKnownIds() {
+        return viewMap.keySet();
+    }
+
     /**
      * Adds a DataView in this list
      *