The absence of women conductors at the world’s top orchestras is no longer news, but it stands out more every year, as women scale male bastions in business, sports and entertainment. Of the 20 largest orchestras in the U.S., only the Baltimore Symphony has a woman music director: Marin Alsop, who last month made history as the first woman to conduct the Last Night of the Proms concert in its nearly 120-year history.