Improve documentation

[serbinsh]

I have been using the package to create some idealized response surfaces and I realized that the documentation isnt complete. For example for many of the functions the output of the function isnt documented. That is the data columns/outputs and units

I also noted that its not always clear if we are using different models for gs etc if they are assuming the model-specific units for g0 etc or a standard unit that then gets converted in the function, as needed.

[JulienLamour]

Sorry if that's not clear. The units are always mol m-2 s-1 in the conductance models. I think this is detailed in the help of the f.make.param function.

[JulienLamour]

This is also detailed in the help of the f.gs function