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.h b/utils/tracers/l2-rate-tracer.h
index 8dba680..bdf2263 100644
--- a/utils/tracers/l2-rate-tracer.h
+++ b/utils/tracers/l2-rate-tracer.h
@@ -61,6 +61,15 @@
   static void
   InstallAll (const std::string &file, Time averagingPeriod = Seconds (0.5));
 
+  /**
+   * @brief Explicit request to remove all statically created tracers
+   *
+   * This method can be helpful if simulation scenario contains several independent run,
+   * or if it is desired to do a postprocessing of the resulting data
+   */
+  static void
+  Destroy ();
+
   void
   SetAveragingPeriod (const Time &period);