model: Add method that is necessary for python bindings

Refs: #2341
diff --git a/model/ndn-l3-protocol.cpp b/model/ndn-l3-protocol.cpp
index dc487bc..39ce2ca 100644
--- a/model/ndn-l3-protocol.cpp
+++ b/model/ndn-l3-protocol.cpp
@@ -351,5 +351,13 @@
   return nullptr;
 }
 
+Ptr<L3Protocol>
+L3Protocol::getL3Protocol(Ptr<Object> node)
+{
+  Ptr<L3Protocol> retval = node->GetObject<L3Protocol>();
+  NS_ASSERT_MSG(retval != nullptr, "L3Protocol is not aggregated on this object");
+  return retval;
+}
+
 } // namespace ndn
 } // namespace ns3