City Plot Recognized, Accurate Happiness

Yields for the city plot are now calculated properly (eg. the AI will
now consider settling on a plot with no yield because it now knows about
a city's minimum yield), happiness values from resources are now
calculated properly, happiness values from natural wonders are now
considered, hardcoding for increased plot priorities for certain
civilizations has been recoded and un-hardcoded, other miscellaneous
tweaks.