Fachbereich Physik, Universität Siegen, 57068 Siegen, Germany;
klose(at)physik.uni-siegen.de, marculesu(at)physik.uni-siegen.de
Received 29 August 2002, revised 2 February 2003,
accepted 6 February 2003 by K. Fredenhagen
CERN, Geneva; dominik.schwarz(at)cern.ch
Received 26 March 2003, revised 11 April 2003,
accepted 14 April 2003 by F. W. Hehl