Bugfix:
- correct number of turns estimation for Culture Diffusion Rate
- fix culture update around a city on capture/gift
- fix plot flipping conditions from citadels and adjacent plots ownership
- nearest city can be NULL if Culture Diffusion range is set too high
- check if a plot is really owned before calling teams functions between owners...
- units will force culture update when entering an enemy plot during a war, even if the plot can't flip.