Fixed minor biomass calculation bug.

When waterFraction < 1 (less water available than requested by crop),
biomass already grown this tick has (1 - waterFraction) of that new
growth removed.  System malfunction, simulated via myProductionRate,
is applied to the growth of myCurrentDryBiomass, but not to its
waterFraction < 1 removal, causing excessive removal under conditions
of simultaneous low water and malfunction.  In extreme cases, this
could drive myCurrentDryBiomass negative.

* Apply myProductionRate scaling to (1 - waterFraction) removal of
  myCurrentDryBiomass grown this tick during low water conditions.