Implementing content store using Boost.MultiIndex container

Adding .SetGroupName("Ccnx") call to Ccnx-related objects (probably
missed some).  This call is advisory, but it is nice to give users
information about Ccnx-related objects (this information is accessible
via --PrintGroup=Ccnx command line argument)
diff --git a/model/ccnx-interest-header.cc b/model/ccnx-interest-header.cc
index 02ad9af..330e2f4 100644
--- a/model/ccnx-interest-header.cc
+++ b/model/ccnx-interest-header.cc
@@ -40,6 +40,7 @@
 CcnxInterestHeader::GetTypeId (void)
 {
   static TypeId tid = TypeId ("ns3::CcnxInterestHeader")
+    .SetGroupName ("Ccnx")
     .SetParent<Header> ()
     .AddConstructor<CcnxInterestHeader> ()
     ;