INSTRUCTIONS: Part 1 contains the text in PS format. Part 2 contains Figs. 3,4,5,6,7a,7b,8,9 (in this order) in PS format INFORMATION: This paper (preprint DESY 92-070) is the author's contribution to the 31. IUKT held in Schladming/Austria February 2-8 1992. It will appear in the proceedings from Springer Verlag. People having problems reproducing the paper from this source, may send electronic mail to me (Marcus Speh): marcus@apollo.desy.de (INTERNET) or I02MSP@DHHDESY3 (BITNET) BODY