The PBS series, “Downton Abbey” premieres this week, and it’s one in a long line of very popular programs that the network has produced. How do they find shows like “Downton,” “Sherlock,” “Brideshead Revisited,” and “Poldark”? What goes into choosing which programs make it on the air? And how has the format for “Masterpiece” affected commercial network programming through the years? We talk to the producer of the series and also to a TV expert about these issues.