The animation wizards behind Wallace & Gromit and Shaun the Sheep are back with a sequel to their 2000 hit feature, “Chicken Run.” Correspondent Seth Doane visits the Bristol, England studios of Aardman Animations, where artists have painstakingly filmed “Chicken Run: Dawn of the Nugget,” a comic adventure in which daredevil chickens seek to rescue their precious hen from a dastardly factory farm. (Think “Chicken: Impossible.”)