This work was sponsored by a Lavoisier Grant;
French Foreign Affair Ministry;
Paris;
France and EU Contract ERBCHBICT 941607 (G.R.): CNRS. CNRS-NSF Exchange Programme AI-06-93 and DOE Grant DE-FG02-91ER40688;
Task A;
Brown University. G.R. wishes to thank LPTHE Paris VI for its hospitality.J .A. thanks 0. Babelon for discussions;
and Brown University Physics Department for their kind support;