"ns mcast.tcl": a multicast routing demo using nam, illustrating
"flood and prune" multicast routing.

At time 1.0 a CBR source at node 1 starts sending to group A (black
packets).  Prune packets are sent (purple).
At time 1.1 a CBR source at node 3 starts sending to group B (red
packets).
At time 1.2 node 2 joins the group B, then leaves at time 1.25.
(Graft packets are off-white.)
At time 1.3 node 2 again joins group B.
At time 1.35, node 2 joins group A.
