adds Budget::Group model to group headings

many refactors through budget related models

app/models/budget/heading.rb

@@ -1,11 +1,11 @@
 class Budget
   class Heading < ActiveRecord::Base
-    belongs_to :budget
+    belongs_to :group
     belongs_to :geozone
 
     has_many :investments
 
-    validates :budget_id, presence: true
+    validates :group_id, presence: true
     validates :name, presence: true
     validates :price, presence: true
   end