use of nested namespaces for modules

This is responding to a comment from Peter in !1026 (merged)

But please don't just dump random symbols in namespace ns3. This module already does way too much of that.

• Can we get a consensus on whether modules like lr-wpan should start to use their own nested namespace, such as olsr and aodv do?
• Should a nested namespace be recommended practice for all future (contrib or mainline) modules? If not, what are the suggested criteria?