The nation’s new postmaster general says the Postal Service is looking beyond its regular mail and package delivery for new sources of revenue. That means building on what Megan J. Brennan calls its “core competency”—the delivery of goods. So what is the agency doing? “We’re testing grocery delivery in San Francisco,” Brennan said in an interview with The Associated Press. “We’re also delivering water, cases of water in Manhattan and the boroughs” of New York City. …