No matter what genre you’re writing for, adding a cello can increase the melancholy. It’s true now, and it was true a couple of centuries ago when J.S. Bach was writing his suites for unaccompanied cello. Journalist Eric Siblin had written about rock for years, but he tells Jim Fleming about his discovery of Bach, and how he came to write “The Cello Suites: J.S. Bach, Pablo Casals, and the Search for a Baroque Masterpiece.”