Cambiar Precision a Campos de Factura a un Valor no dependiente de la precision del Sistema

Redefined account.invoice fields:
* amount_untaxed
* amount_tax
* amount_total
* residual

and in modelo account.invoice.line
* price_subtotal

Being set these fields to a precision of two decimals it is warranted that the account moves for purchase and sale invoices won't trouble when created due to rounding difference.