[IMP] account_voucher: make the field tax_amount always readonly

[IMP] account_voucher: make the field tax_amount always readonly

The field tax_amount is fieled with onchanges and the compute_tax method. Setting a different value than the one computed by the system may lead to unbalanced move (which is obviously wrong).
In the future, handeling these operations by setting the correct value to the tax accounts would be better.