of Leicester, England, and the EU Network "Non-commutative Geo-
metry" at NTNU Trondheim, Norway. The collaboration of the first
and third authors was supported by the Sonderforschungsbereich 288,
"Differentialgeometrie und Quantenphysik" at TU Berlin, Germany,
and by the EU Network and NTNU Trondheim. The third author is
supported by the Sonderforschungsbereich 288 "Differentialgeomet-
rie und Quantenphysik" at TU Berlin. All three authors are most
grateful for the financial help received from these various sources.