By Josh Smith SEOUL (Reuters) -North Korea may be preparing to launch a new submarine believed to be capable of firing ballistic missiles, a U.S.-based think tank reported on Thursday, citing commercial satellite imagery. Images of the Sinpo South Shipyard, on the east coast of the country, from Sept. 18 revealed six barges and vessels gathered around the construction hall quay, said 38 North, which monitors North Korea. “While barges and a dry dock have been occasionally observed around the submarine launch quay at the main construction hall, the presence of six vessels and barges in this are…