[1] CNR, Ist Applicaz Calcolo M Picone, I-00185 Rome, Italy
[2] Natl Inst Nucl Phys, Sect Roma Tre, Rome, Italy
[3] Inst Hautes Etud Sci, F-91440 Bures Sur Yvette, France
[4] Max Planck Inst Gravitat Phys, Albert Einstein Inst, D-14476 Potsdam, Golm, Germany
[5] Univ Southampton, Math Sci, Southampton SO17 1BJ, Hants, England