For the most part, Forbes’ latest billionaire list is a who’s who of New York’s hedge fund managers and the West Coast’s tech giants, with a few oil barons and other old-money types sprinkled in for good measure. But the D.C. area is actually home to two of the 25 richest Americans, and six of the 10 richest counties. Forbes did not provide pictures of Forest Mars, Jr. or John Mars, but does list…