ROME - Scientists in Italy believe they have uncovered a murder — 400 years after it is thought to have taken place. Historians have long suspected that Francesco de' Medici, Grand Duke of Tuscany, and his second wife, Bianca Cappello, did not die of malaria but were poisoned — by Francesco's brother, Cardinal Ferdinando de' Medici, who was vying for the dukedom. For four centuries that theory remained just that — a theory