[Mini-NDN] IP-Routing Helper Fix

[Philipp Moll]

Hi everyone,

I just pushed a fix for the IP-Routing helper to Gerrit. The old version 
of the IP-Routing helper does not work for large topologies, such as the 
GEANT topology. This can be tested with `sudo python 
examples/ip_rounting_experiment.py topologies/geant.conf`. The new 
version fixes the problems and now, the IPRoutingHelper works in 
(hopefully) all topologies.

I made the path in March, but didn't find time to submit the path at 
that time. Now, I can't remember all the details, but pings were not 
successful when the path between two nodes was not symmetric (different 
paths for A-->B and B-->A). Technically, this should not matter, but for 
some reason, ping made troubles. I rewrote parts of the routing helper 
to prevent asymmetric paths (respectively asymmetric subpaths) and now 
pingall results in 100% success (`Results: 0% dropped (1980/1980 
received)`) for the GEANT topology.

Long story short: the patch is working and I think everyone should be 
able to profit from that. However, I am currently busy with finishing my 
PhD, so, please spare me a review with hundreds of patch sets, as I 
experienced it with earlier patches.

Thanks,
Philipp