NePA TesT: Community Network Emulator

Make Ripcord-specific topologies optional

If ripcord.dctopo imports properly, then include its topologies in the
list of available ones. Also replace topo.py with new generic
topologies and update paths.

Move TreeNet to new Mininet API

Also remove all non-object-oriented legacy Mininet code and update
tests.

User-space compatibility is untested, but most of the code for it is
still in.