Philanthropist Denny Sanford made one of the largest gifts in the history of higher education Tuesday, giving $350 million to San Diego’s National University to transform the school into a dominant force among institutions that cater to working adults, especially online. The nearly half-century old university helped pioneer online education and specializes in serving older learners, many who study digitally and at small campuses. But some of National’s degree programs have received low rankings from U.S. News and World Report.