Add single-step leaf-level P-model function

[fabern]

Adds function for single-step leaf-level simulations of the P-model (run_pmodel_onestep_f_bysite).

This is based on the former branch simple_pmodel. This branch should be used instead of the former since it has been rebased at the end of the commit history after the luluc merge.

[fabern]

This currently does NOT give the same result as rpmodel::rpmodel(), see vignette or test-model-runs.R.

• needs to be fixed

[fabern]

The differences between rpmodel::rpmodel() and run_pmodel_onestep_f_bysite() were due to different method_jmaxlim and also different units:

• iwue (Pa vs -),
• rd (mol C m-2 vs. g C m-2), and
• gs or gs_accl (mol C m-2 Pa-1 vs mol C / molPhotons / Pa).

[fabern]

This is now ready for review and merge.

• Besides the functionality for the single-step p-model function, I also:
• double-checkd and corrected many of the code comments specifying units (of ppfd, vcmax, etc...)
• slightly refactored the Fortran-R interface of p-model similar to the current (LULUC-)BiomeE