ANDREA SHALAL U.S. Treasury Secretary Janet Yellen said she had discussed economic sanctions on Russia over its invasion of Ukraine at every stop on her Africa tour and she was hopeful an agreement on the next Russian oil price cap could be reached soon. Yellen is wrapping up a three-country visit to Africa that is aimed at deepening U.S. economic ties with the continent and countering China's long dominance of trade and lending with many African nations. In comments to reporters during a visit to the South African coal-mining province of Mpumalanga, Yellen said the United States was "in the middle of…