model: Finalizing implementation of FaceMetric::Status tracing

Adding test case (ndnSIM-fib-entry), which can be used as an example how
to set up tracing.
diff --git a/model/fw/green-yellow-red.cc b/model/fw/green-yellow-red.cc
index 09f3199..e5d1eae 100644
--- a/model/fw/green-yellow-red.cc
+++ b/model/fw/green-yellow-red.cc
@@ -105,6 +105,19 @@
 }
 
 void
+GreenYellowRed::WillEraseTimedOutPendingInterest (Ptr<pit::Entry> pitEntry)
+{
+  super::WillEraseTimedOutPendingInterest (pitEntry);
+
+  for (ndn::pit::Entry::out_container::iterator face = pitEntry->GetOutgoing ().begin ();
+       face != pitEntry->GetOutgoing ().end ();
+       face ++)
+    {
+      pitEntry->GetFibEntry ()->UpdateStatus (face->m_face, fib::FaceMetric::NDN_FIB_YELLOW);
+    }
+}
+
+void
 GreenYellowRed::DidReceiveValidNack (Ptr<Face> inFace,
                                      uint32_t nackCode,
                                      Ptr<const InterestHeader> header,