Allow biomass inputs as is (instead of representing values at 100% cover and at 360 ppm CO2)

[dschlaep]

Currently,

Input biomass per unit area (g / m2) for each vegetation type represent
values as if that vegetation type covers 100% of the simulated surface.
That way input biomass values are independent of the input composition values.
For example,
  - inputs for forbs: fCover = 0.4, biomass = 300 g/m2
  - inputs for grasses: fCover = 0.6, biomass = 450 g/m2
Then, SOILWAT2 simulates a surface with vegetation of
    fCover = 1 (= 0.4 + 0.6)
and a
    total biomass = 390 g / m2 (= 0.4 * 300 + 0.6 * 450)
of which
    forb biomass = 120 g / m2 (0.4 * 300)
and
    grass biomass = 270 g/m2 (= 0.6 * 450).

We need:

• New input flag "is_biomass_at_cover" (or similar)

  • 0 = "no": input biomass represents values as if that vegetation type covers 100% of the simulated surface (reproducing previous behavior)
  • 1 = "yes": input biomass represents values at the specified cover value (e.g., useful for biomass values measured in the field)

• If "biomass_at_cover" is 1, then don't adjust biomass values for actual cover (or, alternatively, calculate internally biomass as if representing 100 % cover)

• New input to specify reference atmospheric CO2 for which input biomass values are valid (current built-in default is 360 ppm CO2 corresponding which occurred during 1994/1995)

• Adjust effects of atmospheric CO2 concentrations for biomass and WUE adjustments relative to reference concentrations (instead of built-in 360 ppm CO2)

• Fix output BIOMASS_Biomass_tree: it currently does not incorporate correctly updated BIOMASS_Biolive_tree #330