Proposal to simplify document association

https://forums.buildingsmart.org/t/proposal-to-simplify-document-association/3508/2

Right now, the document association concept uses an `IfcRelAssociatesDocument` where the `RelatingDocument` data type is an `IfcDocumentSelect`.

This means that you can _either_ relate a Document Reference, which then further references a Document Information, or you can relate a Document Information directly.

This is confusing to users. Especially confusing is that both references and information may contain a potentially conflicting `Location` attribute.

What are people's thoughts on this? Can this be simplified to change the document select to always require a document reference? Can doc reference and doc information be merged? (I _kinda_ understand how reference may refer to a particular position within the document, so maybe merging isn't the right way...)

Start with the linked forum proposal and the current IfcRelAssociatesDocument, IfcDocumentSelect, Document Reference, and Document Information definitions. Compare the alternatives raised, including Location ownership, and seek an agreed model before changing the specification. Done means the project has chosen and documented a single unambiguous association design.

Written by the indexing model from the issue text.