Added simulation of soil carbon storage and fluxes.  This formulation was
taken mostly from the LPJ model (Sitch, 2003), which in turn used a
Lloyd-Taylor model for the dependence of soil respiration on soil
temperature.  The dependence of soil respiration on soil moisture was
based on the formulation of Yi et al (2012) but modified to allow a small
respiration rate under saturated conditions.

At this point, we do not simulate the storage of carbon in living biomass.
Therefore, the flux of carbon into the soil (litterfall) is set equal to
the total NPP of the previous calendar year, spread evenly over the current
year.  As in the LPJ model, soil carbon is stored in 3 pools: litter (fast),
intermediate, and slow; with associated turnover times of 2.86 y, 33.3 y, and
1,000 y, respectively.  Litterfall enters the litter pool.  Carbon exits the
litter pool through respiration (RhLitter).  A fraction (fAir) of this
respired carbon is in the form of CO2 and is vented directly to the atmosphere
(RhLitter2Atm).  The remainder is sent to the intermediate and slow pools
in the proportions fInter and (1-fInter), respectively.  These pools also
respire carbon, which is assumed to be in the form of CO2 and vented directly
to the atmosphere.

There are several new output variables associated with this feature:
  OUT_RHET: Total heterotrophic respiration vented to the atmosphere
            (= RhLitter2Atm+RhInter+RhSlow)  [g C/m2d]
  OUT_NEE:  Net Ecosystem Exchange (= NPP-RHET) [g C/m2d]
  OUT_LITTERFALL: Flux of carbon from living biomass into litter pool [g C/m2d]
  OUT_CLITTER: Carbon density in the litter pool [g C/m2]
  OUT_CINTER: Carbon density in the intermediate pool [g C/m2]
  OUT_CSLOW: Carbon density in the slow pool [g C/m2]

This feature is part of the carbon cycle, controlled by the setting of the
CARBON option in the global parameter file.