Added an isRunnable() method that returns true if the simulation can be run.

tools/cooja/java/se/sics/cooja/Simulation.java

@@ -1081,6 +1081,15 @@ public class Simulation extends Observable implements Runnable {
     return isRunning && simulationThread != null;
   }
 
+  /**
+   * Return true is simulation is runnable.
+   *
+   * @return True if simulation is runnable
+   */
+  public boolean isRunnable() {
+    return motes.size() > 0;
+  }
+  
   /**
    * Get current simulation title (short description).
    *