ACKNOWLEDGEMENTS
The first author gratefully acknowledges the doctoral studies grant (Grant No. 607796) from CONACyT (Consejo Nacional de Ciencia y TecnologĂ­a) of Mexico. Technology) of Mexico, and the institutional support for a national technical research stay granted by COCyTED (Consejo de Ciencia y TecnologĂ­a del Estado de Durango) is also gratefully acknowledged.