Adding hidden resources

[Sinner]

I finally returned to playing around with hidden resources last night and found that there's a limit of 64 currently allowed by the game engine.

Since we can have far more provinces than that - I believe it's over 200 - that means we can't have province specific units for every province without limiting their numbers, instead having to group provinces by faction, culture, terrain and/or other common features with the occasional restricted hidden resource like Rome or Sparta.