Jimmy Rodwell, 68, convicted in 1980 for a brutal murder in Somerville, is the subject of Casey Sherman’s latest investigation who says he’s behind bars for a crime he didn’t commit. He says the case could shake the reputations of countless law enforcement officers and judges. Sherman talks about the investigation among other topics, including the live-stage adaption of his new book “Helltown,” in the latest episode of Sunday Journal airing this weekend.