(Reuters) - Discover Financial Services, a provider of credit cards, told Reuters it will allow its network to track purchases at gun retailers come April, making it the first among its peers to move ahead with the initiative aimed at helping authorities probe gun-related crimes. The decision came after the International Organization for Standardization (ISO), which decides on the classification of merchant categories used by payment cards, approved in September the launch of a dedicated code for gun retailers. Proponents of the move, including gun control activists and Democratic politicians, say it will allow financial institutions to better assist authorities...