[MERGE] add functionality for approve outdated purchase wh iva documents:
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[REF] unnecessary assignment
[FIX] the server died when trying to cancel a wh doc with no wh lines.
[IMP] add an exeception for withholding date needs to be today or a previous
date.
[FIX] view labels for creating new withholding iva lines.
[IMP] change readonly permissions over 'date_ret' field.
[ADD] outdated wh vat functionality working for purchase wh iva, don't change
anything in the process for sale wh iva.
[MERGE] ovl70-whiva-warning-wh-lines-fortnights | revno 876
[REF] action_date_ret() method now with pep8 and openerp standard.
[FIX] cannot cancel wh iva documents. Now it does (I was clearing the lines and then moves).
[ADD] new 'allow_vat_wh_outdated' boolean field in res.company model.