New function, Area::get_name().
Building now tracks the output of each field (set by the Building_datum, modified by the terrain it's on, and eventually modified by the race that built it).  Move all Building_datum functions to building.cpp.  New function, Building::add_production(); the building_data.cpp macro _production uses this function, which verifies that the amount is > 0 and not infinite.
New function City::add_open_area() adds an area to the open areas vector and does checks (sets it to open, sets up field output, more later).  Used in City::do_turn() when we finish area construction.  Wrote City::get_crops_grown().
City_map::get_farmability() returns the farmability of the tile at the given coordinates.
Tweak farmability of various terrains; make them all multiples of 10 since a farm's default output is 10 (so a farmability of, say, 15 is effectively 10 anyway).