Make getClassName method const

  This virtual methods is defined in Entity class. Changing the prototype
  breaks the overloading mecanism.

include/dynamic-graph/tracer-real-time.h

@@ -54,7 +54,7 @@ namespace dynamicgraph
   {
   public:
     static const std::string CLASS_NAME;
-    virtual const std::string& getClassName ()
+    virtual const std::string& getClassName  () const
     {
       return CLASS_NAME;
     }

include/dynamic-graph/tracer.h

@@ -43,7 +43,10 @@ namespace dynamicgraph
 
   public:
     static const std::string CLASS_NAME;
-    virtual const std::string& getClassName  () { return CLASS_NAME; }
+    virtual const std::string& getClassName () const
+    {
+      return CLASS_NAME;
+    }
 
   public:
     enum TraceStyle