Wrote `getproperty()` and `setproperty!()` for the Animal type

This allows us to get rid of the annoying `animal.traits["traitname"]`
syntax and replace it with the much simpler `animal.traitname`.

src/nature/nature.jl

@@ -13,7 +13,9 @@
     Animal
 
 This is the generic agent type for all animals. Species are differentiated
-by trait dictionaries passed by them during initialisation.
+by trait dictionaries passed by them during initialisation. (Note that each
+trait variable can still be accessed as if it were a normal field name,
+i.e. the trait `phase` can be accessed and modified with `animal.phase`.)
 """
 @agent Animal GridAgent{2} begin
     #XXX is it (performance-)wise to use a dict for the traits?
@@ -22,19 +24,37 @@ by trait dictionaries passed by them during initialisation.
     # to deepcopy the speciesdict.
     traits::Dict{String,Any}
     sex::Sex
-    age::Int32
+    age::Int
 end
 
-#TODO If I write a `getproperty` method for `Animal`, I could get around the ugly
-# `animal.traits[property]` syntax (like Agents.jl does in `model.jl`).
+# Overloading the `getproperty` and `setproperty!` methods for `Animal` allows
+# us to write `animal.property` instead of `animal.traits["property"]`.
+# (This is inspired by Agents.jl in `model.jl`.)
 
+function Base.getproperty(animal::Animal, s::Symbol)
+    if s in fieldnames(Animal)
+        return getfield(animal, s)
+    else
+        return animal.traits[String(s)]
+    end
+end
+
+function Base.setproperty!(animal::Animal, s::Symbol, x)
+    if s in fieldnames(Animal)
+        setfield!(animal, s, x)
+    else
+        animal.traits[String(s)] = x
+    end
+end
+
+    
 """
     animalid(animal)
 
 A small utility function to return a string with the species name and ID of an animal.
 """
 function animalid(a::Animal)
-    return "$(a.traits["name"]) $(a.id)"
+    return "$(a.name) $(a.id)"
 end
 
 """
@@ -44,7 +64,7 @@ Update an animal by one day, executing it's currently active phase function.
 """
 function stepagent!(animal::Animal, model::AgentBasedModel)
     animal.age += 1
-    animal.traits[animal.traits["phase"]](animal,model)
+    animal.traits[animal.phase](animal,model)
 end
 
 """
@@ -150,7 +170,8 @@ such phase, and the conditions under which the animal transitions to another pha
 animal is in the relevant phase. When it is called, it has access to the following
 variables:
 - `animal` a reference to the animal itself. This provides access to `animal.age`,
-    `animal.sex`, and `animal.traits` (a dict that gives access to all species parameters).
+    `animal.sex`, and `animal.<trait>` (where <trait> is a variable that was defined
+    in the top part of the species definition body).
 - `pos` gives the animal's current position as a coordinate tuple.
 - `model` a reference to the model world (an object of type `AgentBasedModel`).
     This allows access to `model.date` (the current simulation date) and
@@ -187,12 +208,7 @@ macro trait(traitname)
     # (i.e. outside of a @phase block). Although this is specified in the documentation,
     # it is unexpected and liable to be overlooked. Can we add a third clause to
     # compensate for that?
-    #TODO replace with `animal.traitname` syntax using `getproperty()`/`setproperty!()`
-    if traitname in fieldnames(Animal)
-        :($(esc(:animal)).$(traitname))
-    else
-        :($(esc(:animal)).traits[$(String(traitname))])
-    end
+    :($(esc(:animal)).$(traitname))
 end
 
 """
@@ -202,7 +218,7 @@ Switch this animal over to a different phase. This can only be used nested withi
 """
 macro setphase(newphase)
     #XXX make this usable in the top part of a species definition?
-    :($(esc(:animal)).traits["phase"] = $(String(newphase)))
+    :($(esc(:animal)).phase = $(String(newphase)))
 end
 
 """