Make refineries unload harvesters faster

The values for loading and unloading time for harvesters are swapped.
This will keep the total amount of time for the harvest/unload cycle, but they will spend less time at the refinery. This will allow a refinery to support 3 harvesters operating from a nearby field, instead of the current 2. 

One effect of this is that it will make buying a factory early game a bit more economical  (though still a bit less so than buying only refineries with no factory), since more harvesters can be used per refinery, increasing demand for harvester production.