Area::amount_produced() tells us how much of a given resource is produced.
Building_datum::amount_produced() and Building::amount_produced() tell us how much of a given resource is produced.
Reduced workers at sawmill from 10 to 5.
City::get_map_info() returns City_map::info() plus info on the area here, if any.  City::draw() draws closed (but fully contructed) areas with a gray background.  City::do_turn() has better comments and produces wood from sawmills.
Use city->get_map_info() instead of city->map.get_info() when getting info on a tile.  Assume mineral_amount_ranking() doesn't end in a space.  Use actual tree count for sawmill version of Interface::display_area_stats().
Terrain_type tracks the terrain to change to when wood is cleared.  New function, Map_tile::clear_wood(), performs this change.  Improve Map_tile::get_info().
Make sawmills buildable in more places.  Include wood_cleared data for forest.  Data for new terrain, TER_BARREN (cleared forest).
New function, trees_amount_ranking(), gives a text description of a quantity of trees.  Don't end mineral_amount_ranking() return values with a space.