Added simulation of photosynthesis.  The photosynthesis formulation was
taken from the BETHY model (Knorr, 2000), which in turn used the Farquhar
model for C3 plants and the Collatz model for C4 plants.  In addition,
inhibition of photosynthesis under saturated conditions (as described
by Frolking et al, 2002) is allowed for.

This feature requires several new veg parameters to be in the veg
library file:
  Ctype:          Photosynthetic pathway; can be C3 or C4
  MaxCarboxRate:  Maximum carboxlyation rate at 25 deg C (mol(CO2)/m2s)
  MaxETransport:  Maximum electron transport rate at 25 deg C (mol(CO2)/m2s) (C3 plants)
  CO2Specificity: CO2 specificity at 25 deg C (mol(CO2)/m2s) (C4 plants)
  LightUseEff:    Light-use efficiency (mol(CO2)/mol(photons))
  NscaleFlag:     TRUE = nitrogen-scaling factors are applicable to this veg class
  Wnpp_inhib:     Moisture level (fraction of maximum moisture) above which photosynthesis experiencing saturation inhibition, i.e. too wet for optimal photosynthesis; only applies to top soil layer
  NPPfactor_sat:  Photosynthesis multiplier (fraction of maximum) when top soil layer is saturated

There are several new output variables associated with this feature:
  OUT_GPP:  Gross primary productivity [g C/m2d]
  OUT_RAUT: Autotrophic respiration [g C/m2d]
  OUT_NPP:  Net primary productivity [g C/m2d]
  OUT_APAR: Absorbed PAR [W/m2]

By default, this feature is turned off.  To turn this feature on, set
CARBON to TRUE in the global parameter file.

When this feature is turned on, you can choose to compute stomatal
resistance via the Jarvis formulation (the formulation used by all
previous versions of VIC) or as a function of photosynthetic demand.
This is determined by the setting of RC_MODE in the global parameter
file.  A value of RC_JARVIS (which is the default) selects the Jarvis
formulation.  A value of RC_PHOTO selects the photosynthetic demand
formulation.