José García López was sweating profusely as he stirred a cauldron of boiling oil in which dozens of potatoes were swimming, though his mind, he said, was on something else.“When I’m done I’m going to buy the paper and candles that I need; I can’t let my grandparents down,” he said Friday afternoon, referring to the decorations he’s going to make to honor his deceased relatives, as he spoke on the sidewalk of the Panteón Francés de la Piedad, an old cemetery in Mexico City.