Use potential biomass instead of potential leaf area for the computation of the C demand

[VEZY]

Currently the C demand is computed with the following steps:

1. we compute the final potential area, the potential leaf area (no stresses) of a fully developed leaf at a given palm age
2. we use it as the maximum parameter for a dynamic potential area of a leaf according to the thermal time elapsed since initiation. This one is a curve that increases with :TT up until the final potential area.
3. we use the derivative of the potential area curve on day i (using the cumulated :TT) to compute the potential increment in leaf area for day i.
4. we modulate this potential increment with stresses
5. we use the sla to compute the carbon demand of the leaf needed to get this potential increment

So at the time, the carbon demand needs the potential area. But the area of the leaf will be computed by VPalm soon (it is an output of the model). So what we propose is to use the biomass for computing the carbon demand instead of the area.

The computation would be the same, except that we would replace area by biomass:

1. we compute the final potential biomass, the biomass of a fully developed leaf at potential (no stresses) at a given palm age
2. we use it as the maximum parameter for a dynamic potential biomass of a leaf according to the thermal time elapsed since initiation. This one is a curve that increases with :TT up until the final potential biomass.
3. we use the derivative of the potential biomass curve on day i (using the cumulated :TT) to compute the potential increment in biomass for day i.
4. we modulate this potential increment with stresses
5. This is the carbon demand already

Then the leaf area is either computed by VPalm (it is a result of the model), or by a model that computes it using the sla.