Add production support to Lua API, port gdi04a, gdi04b and intervention missions

I'm not a fan of this method name.  Could we remove the method above, and call this one `Build` instead?  I don't think its unreasonable to expect scripts to call e.g. `{ "e1", "e1", "e1" }` if they want to build multiple of a unit, as this is how the reinforcements code already works.