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/ndn-l3-aggregate-tracer.cc b/utils/tracers/ndn-l3-aggregate-tracer.cc
index c6fc5e3..5a2d662 100644
--- a/utils/tracers/ndn-l3-aggregate-tracer.cc
+++ b/utils/tracers/ndn-l3-aggregate-tracer.cc
@@ -50,6 +50,12 @@
 }
 
 void
+L3AggregateTracer::Destroy ()
+{
+  g_tracers.clear ();
+}
+
+void
 L3AggregateTracer::InstallAll (const std::string &file, Time averagingPeriod/* = Seconds (0.5)*/)
 {
   using namespace boost;