1. University of Montpellier and CNRS UMR5235 LPHI, Montpellier, France.
2. Rey Juan Carlos University, Madrid, Spain.
3. University of Bonn, Bonn, Germany.
4. Corresponding author. University of Montpellier and CNRS UMR5235 LPHI, Montpellier, France.
5. University of Oxford, Oxford OX1 4BH, UK.
6. University of Kassel, Kassel, Germany.
7. CNRS, INRIA, and the University of Lorraine, Nancy, France, and Max Planck Institute for Informatics, Saarbrücken, Germany.