Add concept of humidity to soil and enable watering each cell

Watering enhances growing speed of both weeds and crops.  Plowing
reduces humidity in the soil.  The player have to decide whether to
plow or to water.
Rename cultivate to plow

The word "cultivate" have too generic meaning.
Agricultural terminology have nuances that are hard to learn,
especially for non-native speakers.  I'm not confident about the
choice of the new word, but it looks better than before.