Comments; AI_city now tracks its control radius (same purpose as the player's city), and will use it when determining its resource production.  More work on AI_city::setup_resource_production().
Move the City_map from Player_city to City.  New function, City::generate_map(), generates our City_map based on a given World_map.  Use this in main().
In Kingdom::add_city() check that world is not NULL.  Generate the new city's City_map.
New mineral, MINERAL_GEMS.  Represents a variety of gemstones which will eventually be important for magical purposes.