CSV format versioning

[timo95]

We have done multiple changes to the CSV format in the past and keeping track of these will only get more difficult when we change it again. Also the error messages when reading an incompatible version aren't very user friendly. A simple version file in the graph directory with a single number would IMHO be enough to track future versions. It would also provide us with the possibility to automatically choose the correct legacy parser when reading graphs in older versions.