When a Nickel Opened Doors; The Automat's Golden Age, Before Coffee Soared to 10 Cents
New York City automats recalled in light of exhibition at Museum of City of New York and companion book, The Automat: The History, Recipes and Allure of Horn & Hardart's Masterpiece, by Lorraine B Diehl and Marianne Hardart, great-granddaughter of co-founder; photos (M)