Provide identity to IFC files

Description of the proposal:

IFC files don't have persistent identity. When a file is generated from a software, it is nearly impossible, for example, to know that the next IFC file of the same project is related to the previous version. This causes challenges, for example, in dealing with BCF. When a BCF Topic is sent to a CDE, it is impossible to reliably identify the files that the topic is dealing with. BCF tries to send the potential identifiers, like IfcProject GUID, filename, timestamp, etc, but none of these has been proven reliable.

The proposal is to add to the IFC headers the following entries:

* DocumentID: a UUID that persists throughout the versions of the file that are generated from the same model
* DocumentRevision: a version-specific UUID

Is this a proposal to 'add', 'remove' of 'change' entities in the schema (pick one):

What do we win: Reliable identification of IFC files

What do we loose

Schema impact: None

Instance model impact: None

Backwards compatible: Yes

Automatic migration possible: No

Additional implications: Implementing software should have a provision for the user to reset the Document ID, for example, when file is saved as.