slightly refactored display? code in chainable

git-svn-id: https://dev.finn.de/svn/cockpit/branches/reporting_merge@1605 7926756e-e54e-46e6-9721-ed318f58905e

app/models/cost_query/chainable.rb

@@ -201,11 +201,12 @@ class CostQuery < ActiveRecord::Base
       end
     end
 
-    inherited_attributes :db_field, :display
+    inherited_attribute :db_field
     def self.field
       db_field || name[/[^:]+$/].underscore
     end
 
+    inherited_attribute :display, :default => true
     def self.display!
       display true
     end