Use Building::get_maintenance() in Area::get_maintenance().
New function, Building::get_maintenance().
Adjust upkeeps, since they're now measured in 1/10ths of a gold.
Remove City::interface_buildings().  Treat building upkeep costs as being 1/10th of a gold.
Replace num_maintenance with text_upkeep, since we need to divide the number by 10.
Fix problem with advancing the month too early (and winding up on day 0 as a result).
Format building upkeep as 1/10th of the reported value (e.g. int(13) => std::string("1.3")).