University of Szczecin
Received 3 April 2000, revised 26 June 2000, accepted 28 July 2000 by F. W. Hehl
Institut für Theoretische Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe; vondelft(at)tfp.physik.uni-karlsruhe.de
Received 1 August 2000, accepted 22 August 2000 by U. Eckern
(approx. 60 pages)