[1] Inst. d'Info. i Aplicacions, Universitat de Girona, Campus Montilivi, E-17071 Girona, Spain
[2] Dept. of Applied Mathematics III, Technical University of Catalonia, Campus Nord, E-08034 Barcelona, Spain
[3] Dept. of Appl. Math. and Comp. Sci., Universitat de Girona, Campus Montilivi, E-17071 Girona, Spain
[4] Inst. d'Info. i Aplicacions, Universitat de Girona, Campus Montilivi, E-I7071 Girona, Spain