Could someone explain the special resource improvements that are available. Specifically, It seems that they get "outdated" quickly. Am I looking at this correct? For example...the Durantium Refinery gives a 20% boost to manufacturing and a +2 adjacency bonus. The first improvement, the Solar Plant gives 25% and also a +2 to adjacent tiles. With limited tile space, why would I ever build the Durantium Refinery? What about the other 4? Any suggestions?