1. a Foundation for Research and Technology--Hellas , Institute of Applied and Computational Mathematics, Regional Analysis Division , FORTH--IACM, Vassilika Vouton, PO Box 1527, Heraklion, Crete, GR-71110, Greece E-mail:
2. b Department of Applied Physics , University of Athens , Building PHYS-V, Athens, GR-15784, Greece E-mail: