1. Information Technology Institute, Riga Technical University, LV-1048 Rīga, Latvia
2. Computing Sciences Department, School of Technology and Innovations, University of Vaasa, FI-65200 Vaasa, Finland
3. School of Electrical and Computer Engineering, National Technical University of Athens, 157 73 Zografou, Greece
4. OFFIS—Institute for Information Technology, 26121 Oldenburg, Germany