By Lucila Sigal BUENOS AIRES (Reuters) – Argentine President Alberto Fernandez called for unity on Saturday as protesters marched in the capital to the gates of the presidential palace, lambasting his government over soaring inflation and a crushing national debt. The center-left president is facing a rising challenge from a militant left-wing of the ruling coalition that wants more state spending to ease high poverty levels and inflation. Two key moderate allies have left his Cabinet in the last month. The South American country, a major producer of soy and corn, is grappling with inflation r…