add version info / switches for file formats in readers/writers?

[dglaeser]

Currently, the writers write the latest file format version and one cannot select the version to be written (e.g. an older one). Also, the readers cannot be configured for a specific file format version at the moment. We may introduce versioning in both readers and writers? On the other hand, the current readers simply read what they find and should be happy with both older and newer versions.