1. Helmholtz Institute Freiberg for Resource Technology, and Institute for Stochastics; TU Bergakademie Freiberg; 09599 Freiberg Germany
2. Dept. Matemática Aplicada III; Univ. Politècnica de Catalunya; 08034 Barcelona Spain
3. Dept. Computer Science, Applied Mathematics, and Statistics; Univ. de Girona; 17004 Girona Spain