C. Barrabès: Research Fellow previously: Département d'Astrophysique Relativiste et Cosmologie, CNRS Observatoire de Paris-Meudon and Laboratoire de Mathématiques et Physique Théorique, UMR 7350-CNRS Université de Tours.
; P. A. Hogan: Research Fellow previously: School of Theoretical Physics, Dublin Institute for Advanced Studies, Center for Relativity, University of Texas at Austin and School of Mathematics, Trinity College Dublin.