[dm-aggregates] Use Property#load to load the values from the datastore

* This change allows a normal and custom property to be used
  interchangeably
* Previously used Property#typecast, which is primarily geared towards
  taking external input and converting it into an internal representation.

[#584 state:resolved]

dm-aggregates/lib/dm-aggregates/adapters/data_objects_adapter.rb

@@ -27,7 +27,7 @@ def aggregate(query)
                 if field.respond_to?(:operator)
                   send(field.operator, field.target, value)
                 else
-                  field.typecast(value)
+                  field.load(value)
                 end
               end
 
@@ -48,19 +48,19 @@ def count(property, value)
       end
 
       def min(property, value)
-        property.typecast(value)
+        property.load(value)
       end
 
       def max(property, value)
-        property.typecast(value)
+        property.load(value)
       end
 
       def avg(property, value)
-        property.type == Integer ? value.to_f : property.typecast(value)
+        property.type == Integer ? value.to_f : property.load(value)
       end
 
       def sum(property, value)
-        property.typecast(value)
+        property.load(value)
       end
 
       module SQL