lr-wpan: (fixes #1130) Delay channel creation and add methods to set propagation loss and delay models in LrWpanHelper

- Delay channel creation by moving it to Install()
- Add SetPropagationDelayModel() and AddPropagationLossModel() methods to allow setting propagation loss and delay models in LrWpanHelper
- Update all examples that use LrWpanHelper to reflect these changes

examples/ipv6/wsn-ping6.cc

@@ -74,6 +74,8 @@ main(int argc, char** argv)
 
     NS_LOG_INFO("Create channels.");
     LrWpanHelper lrWpanHelper;
+    lrWpanHelper.SetPropagationDelayModel("ns3::ConstantSpeedPropagationDelayModel");
+    lrWpanHelper.AddPropagationLossModel("ns3::LogDistancePropagationLossModel");
     // Add and install the LrWpanNetDevice for each node
     // lrWpanHelper.EnableLogComponents();
     NetDeviceContainer devContainer = lrWpanHelper.Install(nodes);