Create dummy material if needed. In newer versions of ROOT (issue was with 5.34/17) also TGeoVolumeAssembly has a medium assigned. Since this medium is neither existing in the GeoManager at time it is needed, nor it is defined in the ASCII media files, it is create on the fly when needed. The same procedure as in TGeoVolume::CreateDummyMedium() is used. This change works at least with ROOT version 5.34/13 and 5.34/17. These are the version used in dec13 release of FairSoft and the master branch in the git repository at this time. The commit fixes #4.