For years this cold creamed leek and potato soup created by Louis Diat, a famous French chef from the old Hotel Ritz Carlton in New York City, was a favorite spring and summertime first course or light supper. Gradually, as my taste sensibilities changed, I no longer felt comfortable about supping on so much cream. But then I eventually started longing for vichyssoise again and discovered that the richness of the flavor comes more from the potatoes and a good-qual