Added explicit heterotrophic bacterial biomass:

* Constant stoichiometry of C:N of 5:1
* Consume DOC and DON and convert these to DIC and NH4
* Consume DOC and DON in their environmental ratio, which is often > 5:1
* Thus, heterotrophic bacteria supplement growth by taking up NH4 and compete directly with phytoplankton
* Heterotrophic bacteria are also facultative, in that they can dynamically convert to using NO3 as an electron acceptor under low oxygen
To do:
* Make heterotrophic bacteria a part of the grazing by microzooplankton
* Improve their competitive interaction with phytoplankton for NH4 (currently no uptake kinetics (i.e. no K))
* Include iron requirement as well, but because Fe is separate from DOM, it can be treated separately.

Author: pearseb