Funny the images that stay with you from childhood. Any time someone mentions the Civil War, there’s this snapshot that flits across my brain of being in my parents’ TV room watching the Ken Burns documentary on the subject. It’s nothing more specific than that, but I remember my whole family being glued to the TV for days. I was 12 when “The Civil War” came out. Four years later, Burns had us fixated again with his nine-part series on the history of baseball. He’s done much since then, including an acclaimed history of jazz, but nothing that really pushed the Thompson family buttons. That is, until this past week when “The National Parks: America’s Best Idea” came to PBS.