Add Bianchi validation script

Over the years, people have written simulation script to validate ns-3 wifi against the well-known Bianchi model. This has never been included in ns-3 so far. Recently, we discovered a regression (#152 (closed)) where ns-3 was highly deviating from the Bianchi model (and the implementation of the model itself was also not correct). We have thus decided to rework the existing script to run ns-3 against Bianchi, and add this to our regression.