[tests/examples] Use real data for testing and examples

[nemesifier]

The good @mwarning has provided us some real data sets coming from the Freifunk communities:
https://github.com/mwarning/meshnet-lab/blob/master/data/freifunk/

We want to use this in two way:

1. use the big data sets to stress test the library
2. use the smaller datasets for examples (we have to remove any sensitive info)

I will do some testing to prepare the datasets asap.

[mwarning]

The data from Freifunk Leipzig seems to be rather diverse in structure (~300 nodes). The data from Freifunk Munich is the biggest set (~1800 nodes). Both seem to be good fit.

[nemesifier]

@mwarning we could try them in different examples to stress test the library a bit.