Clear natural improvements

[kjniemela]

Natural "improvements", such as forests, should be able to be cleared by the builder - maybe this should take some amount of time, and maybe it should yield resources - although it's unclear how exactly this would work.

[clniemela]

It should yield lumber and take a single turn. Introduce lumber resource.

[kjniemela]

We've generally tried to avoid "specific" resources, and instead stuck to more general resources like "food" and "production"... But in either case, the main issue is where the yielded resources should be stored, since this clearing the forest would leave the tile with no improvement on it to store anything in.