By THÉRÈSE MARGOLIS     Just finding Nom Gastronomía Expuesta restaurant is no easy feat. Tucked away in a back alley behind the Dumas Marché food mall in Mexico City’s Colonia Polanco, there are few clues other than a dimly lit copper lantern and a tiny hand-painted sign on a storeroom door to help you locate its whereabouts. Part of the reason for