make sql aggregation inherit from its Report correspondent

app/models/cost_query/group_by/sql_aggregation.rb

@@ -1,18 +1,10 @@
 module CostQuery::GroupBy
   module SqlAggregation
-    def responsible_for_sql?
-      true
-    end
-
-    def compute_result
-      super.tap { |r| r.important_fields = group_fields }.grouped_by(all_group_fields(false), type, group_fields)
-    end
+    include Report::GroupBy::SqlAggregation
 
     def sql_statement
       super.tap do |sql|
-        define_group sql
         sql.sum :units => :units, :real_costs => :real_costs, :display_costs => :display_costs
-        sql.count
       end
     end
   end