Check ipcc history files

[luyssaert]

There are several inconsistencies between the stomate history files and the ipcc history files:

• The npp of plant components (leaves, roots, ...) is written to the ipcc history files. It should be checked whether the sum of these components adds up to the npp written to the stomate files. Based on a quick look at the way it is calculated this is not the case. The npp written to stomate seems to be correct as it includes all its components. The npp written to the ipcc history files is inconsistent because its calculation is based on bm_alloc but the way bm_alloc is calculated ignores the C and N allocated to the reserves and labile pool. A first easy check could be to look at the bm_alloc values for the carbres and labile pool. If those are zero there is certainly a problem. If they are not zero then consistency should be formaly checked in the code.
• gpp, gpp_ipcc2, gppgrass, gppcrop, gpptree are written in sechiba.f90 and do not account for the bypass of C and N uptakle through the variable atm_to_bm. This variable contains the C and N that ORCHIDEE needs to construct a sapling because it cannot grow from a seed. In reality this C will enter the plant through GPP. This flux is especially important for crop PFTs because every year atm_to_bm is used to plant crops.
• In stomate_lpj.f90 several AR6 variables are calculated. These variables were not documented and when merging the code they could not be reconstructed. Check the definition and use the ORCHIDEE 4.0 variables to recosntruct the calculation of flulccresiduelut, flulccresidue, flulccatmlut, and flulccproductlut.