Documentation improvements for BiomeE

[pepaaran]

Here I just list some possible improvements for the BiomeE implementation:

• Set nyeartrend and firstyeartrend to match the available forcing data. Potentially, also n, n_day and n_year, which are passed to the Fortran routine.
• In the "BiomeE usage" vignette, describe how the drivers for Leuning and P-model approaches differ (e.g. time scale, params_siml$method_photosynth...). This information is lacking.
• In run_biomee_f_bysite.R there should be a check for correct model parameters and simulation dates. Currently it only checks whether forcing variables exist. Parameter-data matching should be checked.
• Replicate documentation of functions from P-model, including data and input variables to all functions.