Add cropcover function, fix cropheight return value in case it isn't a farm plot

src/crop/farmplot.jl

@@ -59,6 +59,17 @@ Return the height of the crop at this position, or nothing if there is no crop h
 (utility wrapper).
 """
 function cropheight(pos::Tuple{Int64,Int64}, model::SimulationModel)
-    ismissing(model.landscape[pos...].fieldid) ? nothing :
+    ismissing(model.landscape[pos...].fieldid) ? 0.0cm :  # TODO: 0.0cm correct here?
               cropheight(model.farmplots[model.landscape[pos...].fieldid])
 end
+
+"""
+    cropcover(model, position)
+
+Return the crop cover of the crop at this position, or nothing if
+there is no crop here (utility wrapper).
+"""
+function cropcover(pos::Tuple{Int64,Int64}, model::SimulationModel)
+    ismissing(model.landscape[pos...].fieldid) ? 0.0 :  # TODO: 0.0 correct here?
+              cropcover(model.farmplots[model.landscape[pos...].fieldid])
+end