* Added explicit Fe limitation of heterotrophic bacteria with uptake kinetics

* Included uptake kinetics of NH4 by heterotrophic bacteria
* Thus, competition with phytoplankton now can be managed via varying half-saturation coefficients for Fe and NH4 uptake
* MAJOR FIX to phydoc and diadoc (overflow exudation of DOC):
 - substantially reduced these rates to be at maximum 50% of total NPP (managed via `overflow` parameter)
* Normalized prey preference weights to sum to 1, reflecting ecological theory, specifically regarding "diet fractions"

Author: pearseb