By any metric, WGBH’s “Downton Abbey’’ has hit a home run. Nationally, it has more than doubled PBS’s prime time audience. Locally, “Downton’’ is enjoying a 5.8 rating, twice as high as “Masterpiece Classic’’ ratings last year. So public television has momentarily expanded its audience beyond its core fan base of superannuated, upper class twits - thanks to a superannuated, upper class twit. “Downton’s’’ creator, Sir Julian Fellowes is a life peer in Britain’s House of Lords, lives in a grand manor house in Dorset and is married to a royal lady-in-waiting. Their only child is the Hon. Peregrine Charles Morant Kitchener-Fellowes, to which one can only say: Pip! Pip!