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.