utils/tracers: Adding *::Destroy () method in Tracers

This way it becomes possible to do clean up before applications
terminates (useful if the same scenario is used for several simulation
runs or if there is a need for post-processing of the resulting files)
diff --git a/utils/tracers/l2-rate-tracer.cc b/utils/tracers/l2-rate-tracer.cc
index 48d2589..a4c86b9 100644
--- a/utils/tracers/l2-rate-tracer.cc
+++ b/utils/tracers/l2-rate-tracer.cc
@@ -47,6 +47,12 @@
 }
 
 void
+L2RateTracer::Destroy ()
+{
+  g_tracers.clear ();
+}
+
+void
 L2RateTracer::InstallAll (const std::string &file, Time averagingPeriod/* = Seconds (0.5)*/)
 {
   std::list<Ptr<L2RateTracer> > tracers;