(This is an extendend version of paper number 213)
(This paper has appeared in Int. Math. Res. Notices 2007, no. 16, Art. ID rnm051, 34 pp)
A. Fernández López < emalfer@agt.cie.uma.es >
E. García < esther.garcia@urjc.es >
M. Gómez Lozano < magomez@agt.cie.uma.es >
E. Neher < neher@uottawa.ca >