Make rain give soil humidity rapidly

Rain makes soil wet way faster than dispersion.
Tweak logics and parameters to simulate more realistically

* Corns and potatos have quality that can degrade over time if
  humidity is too high.

* Crop grow speed is no longer proportional to humidity.  Instead,
  humidity around 50% is best for growing.

* Quality now have a threshold at which selling value is affected.
  This threshold is 75%.  This means limited degradation is allowed
  to sell the crop at the highest price.  In exchange, crops degrade
  to some extent even if the environment condition is the best.

* Make soil humidity disperse (approach to atmospheric moisture) faster.
  Watering is required more often.

* Decelerate increasing rate of soil fertility.  The wild life activity
  (namely, feces) is not so drastic.  This makes the game a little
  harder and makes fertilizing more important.