[IMP] account: cash.box.in defines its _columns, do the same for cash.box.out.

This is needed to solve an inheritance problem when adding a superclass
`base.model` (I guess this is related to the fact some code in the orm
does a copy from the class to the instance).

bzr revid: vmt@openerp.com-20130416132621-xo0lckimczqpvyos

addons/account/wizard/pos_box.py

@@ -67,6 +67,8 @@ class CashBoxIn(CashBox):
 class CashBoxOut(CashBox):
     _name = 'cash.box.out'
 
+    _columns = CashBox._columns.copy()
+
     def _compute_values_for_statement_line(self, cr, uid, box, record, context=None):
         amount = box.amount or 0.0
         return {