Louis de Bernieres evoked the romance of the mandolin when he plucked at readers' heart strings with his novel, Captain Corelli's Mandolin. The mandolin is often dismissed as a novelty, but Corelli was a virtuoso who wanted to compose concertos. In Naples, at the turn of the last century, Raffaele Calace wrote over 170 mandolin compositions. Alison Stephens journeys to the workshop where the grandson of the great man still makes mandolins for export. Producer Charlie Taylor (R)