Move definition of Cell from FarmGame instance member to function member

It's more natural place to define.  We may be better using some namespace
rather than defining as a direct member of FarmGame, but we would continue
like this for the time being.
Make potatoes degrade by continuous cropping

You can crop corns in between potatoes to prevent diseases induced
by continuous cropping.  I think it's called rotation in farming
terminology, but I'm not confident.

We had to introduce quality to crops and potatoPest to cells in
order to realize degradation.  The simulated rate of degradation is
suboptimal.  We could insert another delay element to the modulation
of quality for more realistic result.