Set PostOfficeBox when address has a post box in UN/CEFACT 16B-CII

Closes #15005
(grafted from 702f1b938b21c1bbe0536037c300c9c103a952e2)

--HG--
branch : 8.0

modules/edocument_uncefact/template/16B-CII/CrossIndustryInvoice.xml

@@ -25,6 +25,7 @@ this repository contains the full copyright notices and license terms. -->
     </py:def>
     <py:def function="TradeAddress(address)">
         <ram:PostcodeCode py:if="address.postal_code">${address.postal_code}</ram:PostcodeCode>
+        <ram:PostOfficeBox py:if="address.post_box">${address.post_box}</ram:PostOfficeBox>
         <ram:BuildingName py:if="address.building_name">${address.building_name}</ram:BuildingName>
         <py:with vars="lines = (address.street_unstructured or '').splitlines()">
             <ram:LineOne py:if="len(lines) > 0">${lines[0]}</ram:LineOne>