WASHINGTON/LOS ANGELES (Reuters) - U.S. Energy Secretary Steven Chu, the Nobel Prize winner who shepherded an effort to help spur a clean energy U.S. economy, will step down after a tenure rocked by the failures of some costly government investments. Chu's departure, which was announced Friday and follows similar moves by the Environmental Protection Agency administrator and the interior secretary, will allow President Barack Obama to craft a fresh team to address climate change. ...