Proposed attribute to store version added or deprecated per classification reference

Originally a suggestion [here](https://forums.buildingsmart.org/t/additional-attribute-request-for-ifcclassificationreference/838/3) by @jmirtsch

The usecase is that classification systems are released regularly, such as every quarter. An IFC project may start using an old classification as signified by the EditionDate attribute, and each reference is assumed to have the same edition date, since it is part of that classification. However, especially when the classification is part of a central distribution, it is unclear which references within that classification are added / deprecated. To solve this it is proposed that the date the reference was added, or date it was made obsolete should be added as attributes to each individual reference.

Note that in this particular usecase a classification would be used in an IfcProjectLibrary from my understanding, potentially not directly in projects.