Improve baseDir default behaviour

[Kreavita]

I propose setting the veins baseDir to the location of the launch config file (e.g. veins_launchd.xml) by default. It makes more sense to me, because there may be one config file for each scenario, which is where your OSM files should live that get referenced in the config.

This should make using multiple SUMO scenarios with the same networks easier, as you do not need to duplicate your networks, or maintain the baseDir XML tags. In my experience the latter became cumbersome in collaboration, because the basedir tag seemed to not work well with relative paths.

For standard projects, there should be no breaking change, as the network file resides in the same directory as the veins launch config by default.