This work was financially supported by the University of Buenos Aires (UBACyT; TX 117 and X093), ANPCyT (Agencia nacional de Promoción de Ciencia y Tecnología) PICT 10621, the International Program for Physical Science of Uppsala University, Sweden (IPPS), the Research Institute of Universidad Nacional de Ingeniería, the CONCYTEC (Peruvian Research Council), and CYTED network VIII-G.