By THÉRÈSE MARGOLIS What do you get when you cross the peppery-citrusy flavors of regional Yucatan cuisine with the smoky tastes of Argentine gaucho cookery? If you’re lucky, the answer is Sylvestre Artz, an unpretentiously elegant meat house in Mexico City’s southern Colonia Pedregal where spicy Yucatan achiote and habanero chillies are fused with the vaporous earthiness of a Pampas