James Cameron, that visionary mega-director of historic oceanic tragedy and, more recently, virtual blue puma-people, has dug into his giant pockets to option a book for a possible film project that’s quite different in subject and tone from his latest blockbuster, “Avatar.” Variety reported Friday that Cameron now has movie dibs on Charles Pellegrino’s nonfiction book “The Last Train From Hiroshima: The Survivors Look Back.”