Calculates the Outside Option Index proposed by Caldwell and Danieli (2018) < https://drive.google.com/file/d/1j-uwD19S4gqgXIXeYch9jGBCaDhWZlRQ/view>. This index uses the cross- sectional concentration of similar workers across job types to quantify the availability of outside options as a function of workers’ characteristics (e.g. commuting costs, preferences, and skills.)