make a model-specific filter function for DA in ISAM

[viatkin]

Hi Cuijuan!

Your model looks great and we added it to the traceability analysis and model comparison.
To do so, I had to put the computation of fluxes (e.g. r_C_GVF_2_C_AGML) from your original parameters (e.g. f and k) to the source.py.

However, because you are using f and k as parameters in data assimilation, you need a model-specific filter function that would make sure that fs and betas add up to less than 1. Currently your inspect_model.py uses the general helpers function that takes care of betas, but not of fs.

Thanks!