Added alternate units. Some units can now be constructed with an alternate resource or building. The purpose is to make units easier to build and hopefully this will help the AI out. My choices for resources and buildings are based on my research of history.
Stone no longer is a strategic resource. It never was historically as far as I can tell. Honestly I didn't like it as the requirement of only one unit. The Catapult now can be constructed with Iron or Copper since these materials are part of the construction of ancient catapults.
Gunpowder units have the option of requiring a Nitrary instead of Niter for construction. Niter was not just found in nature. It was also "farmed" in a Nitrary.
Classical Era units can now be constructed with Copper or Iron.
Most Modern naval units now have the option of using Oil, Coal, or Uranium.
