fix #170: bugfix in managing repeated elements in the OPEN list of A*

hipster-core/src/main/java/es/usc/citius/hipster/algorithm/AStar.java

@@ -104,7 +104,7 @@ protected N takePromising() {
          */
         public N next() {
             // Get and remove the best node in the queue
-            N current = queue.poll();
+            N current = takePromising();
             S currentState = current.state();
             // Remove from open as well
             open.remove(currentState);