Ever since the first “Thor” film debuted six years ago, the franchise has always been the black sheep of the Marvel Cinematic Universe. “Thor” faltered in trying to combine Earth-based antics with the epic, Shakespearian-style storytelling of the Thor comics of the ’80s. The sequel, “Thor: The Dark World,” tried to add sci-fi elements to this mixture but in doing so lost the heart and humor that made the first movie somewhat enjoyable.