Resource Improvements

Adds a filter to the Trade Panel that shows up when you click a trader, letting you sort the list by a certain resource or yield type you want (say you want Fish for your factory in the Modern era, you could sort for the trade routes with the most Fish). Also adds a toggle so that untradeable cities are still included in the initial sort, meaning you dont have to scroll all way the down to see the best ones if you needed to Improve Trade Relations.