Brief documentation for the current UML encoding of IFC and issues

[BenzclyZhang]

This is not a proposal but information and findings during the process of converting IFC and part of ifcDoc (Pset and Qset) to UML and use it in the IFC Rail and IFC Infra Extension projects.

https://drive.google.com/file/d/1WbVf3cta4A5SiykBBEGJsDo4wIaMlzSM/view?usp=sharing

Issues and remarks are highlighted. Some of the issues have already been discussed by people here.