Add a new DigitalAgreementID BIE to the root of any business document to directly reference an overall Digital Agreement

Any UBL document has a ProfileID and CustomizationID in order to associate a precise profile and customization.
Different profiles and document customizations could be part of an overall Digital Agreement (e.g. an OpenPEPPOL agreement).
Now that UBL has a DigitalAgreement document it could be nice to have the possibility to directly reference a digital e-business agreement from each UBL document instance.