The external document location of a supporting document (BT-124) was
only handled for UBL. The CII parser now reads and the CII writer
writes ram:URIID of an AdditionalReferencedDocument.

UBL has no type code for supporting documents (BG-24), so a UBL
invoice written as CII got an empty ram:TypeCode. The UBL parser now
sets the CII type code 916 for documents without DocumentTypeCode
(the UBL writer does not write it), and the CII writer no longer
writes an empty TypeCode element.

Fixes #195