Add some docstrings and comments

src/crop/cropmodels.jl

@@ -3,6 +3,12 @@
 ### Functions for switchable crop models.
 ###
 
+"""
+    initfields!(cropmodel, cropfile, growthfile)
+
+Initialise the crop model.  Returns the crop types available in the
+simulation, as well as the types `Tcroptype` and `Tcropstate`.
+"""
 function initcropmodel(cropmodel::AbstractString, cropfile::AbstractString, growthfile::AbstractString)
     if cropmodel == "almass"
         Tcroptype = ALMaSS.CropType
@@ -22,7 +28,7 @@ end
 """
     initfields!(model)
 
-Initialise the model with its farm plots.
+Initialise the farm plots in the simulation model.
 """
 function initfields!(model::SimulationModel)
     n = 0
@@ -56,6 +62,7 @@ function initfields!(model::SimulationModel)
     @info "Initialised $n farm plots."
 end
 
+# internal utility function
 function make_cropstate(model::SimulationModel, cropmodel::AbstractString)
     if cropmodel == "almass"
         phase = (month(model.date) < 3 ? ALMaSS.janfirst : ALMaSS.marchfirst)