Create STL_USDA_Low_Income_Low_Vehicle_Access_Food_Deserts.geojson

Census Tracts that are "Low Income and Low Vehicle Access" food deserts according to the USDA Food Access Research Atlas: http://www.ers.usda.gov/data-products/food-access-research-atlas.aspx.

Low Income and Low Vehicle Access: Low-income census tracts where a significant number of households have low vehicle access or a significant number or share of residents are more than 20 miles from the nearest supermarket.