PDME — Add relationship from document master directly to vault

Summary: B) Add a relationship from a DocumentMaster directly to a Vault with the corresponding document
data. If the PdmEnablers are used for importing or exporting a partial model from one Pdm-
System to another, there often will be only one file to import/export, and only in rare cases
complete document-hierarchies will be exposed. With a direct relationship from DocumentMaster
to a corresponding Vault there would be no need to build up the complete hierarchy for
documents (Master, Revision, Iteration, File, Vault) if you want to expose only one single file,
which could eliminate the creation of much overhead.