* Study solves puzzle that eluded scientists for 20 years * Finding should help development of new HIV/AIDS medicines * Allows scientists to see how Merck and Gilead drugs work LONDON, Jan 31 (Reuters) - Scientists say they have solved a crucial puzzle about the AIDS virus after 20 years of research and that their findings could lead to better treatments for HIV. British and U.S. researchers said they had grown a crystal that enabled them to see the structure of an enzyme called integrase, which is found in retroviruses like HIV and is a target for some of the...