Lockheed Martin announced after trading hours yesterday that Louis Hughes, the company's president and chief operating officer, will leave the corporation effective Oct. 31 after only six months on the job. Robert Stevens, the company's executive vice president and chief financial officer, has been named to succeed Hughes. Hughes, who has been in the position only since last April, cited personal and family reasons for his decision. "While very rewarding professionally, the position of president has necessitated that I be...