MEXICO CITY (Reuters) - Heavily armed Mexican police, backed by helicopters, locked down the capital's most notorious neighborhood on Tuesday as part of the latest offensive against rampant drug trafficking. Hundreds of officers with assault rifles lined the Tepito neighborhood's main artery, searching passing motorists. Police had stormed the district, a warren of scruffy homes and market stalls, last week and seized a tenement complex known as "The Fortress" -- reputedly a major cocaine and marijuana distribution center. President Felipe Calderon has sent thousands of troops to fight drug gangs in their strongholds along the Pacific coast and near the...