1. Istituto per le Applicazioni del Calcolo "M. Picone", C.N.R., I-00161 Roma, Italy
2. Sezione INFN di Firenze Polo Scientifico Via Sansone 1, I-50019 Sesto Fiorentino (FI), Italy
3. International Centre for Relativistic Astrophysics, University of Rome, I-00185, Rome, Italy
4. Faculty of Engineering, University Campus Bio-Medico of Rome, via E. Longoni 47, I-00155 Rome, Italy
5. Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 2EG, UK
6. Department of Mathematical Sciences, Villanova University, Villanova, PA 19085, USA