Notes on how farming works - mostly to help me remember, but documentation is nice too!
Remove currently-infinite loop.
New function Map_tile::get_max_food_output() figures out the maximum food output from a single field on this tile.  Presently only used for AI_city to figure out where to farm.
Refactor the way field_output is used.  This includes rewriting Player_city::get_food_production() and the code for getting non-food resources from farms.  The end result is less food lost due to rounding; this means that tile farmability can be a value that is NOT a multiple of 10, and even crappy farmers will get some food!
Update several farmability values.