1. Istituto per le Applicazioni del Calcolo "M. Picone", C.N.R., I-00161 Roma, Italy
2. ICRA International Centre for Relativistic Astrophysics, University of Rome, I-00185, Italy
3. Department of Physics "E.R. Caianiello", University of Salerno, I-84081, Italy
4. Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 2EG, UK
5. Department of Physics, University of Perugia, I-06100, Italy
6. Department of Physics, University of Como, I-22100, Italy