Material simplification

The simplification of materials is cool, but during the implementation we found "strange" that to identify a material, we had to look for "objects with "bsi::presentation::diffuseColor" and hopefully with a "bsi::name".

Could the use of something similar to "bsi::ifc::v5a::class" but for IfcMaterial be more elegant?

Review the current material identification references to bsi::presentation::diffuseColor and bsi::name, then compare them with bsi::ifc::v5a::class. Determine whether an equivalent IfcMaterial identifier is sufficiently specified; done means the proposed approach and required schema changes have been agreed and documented.

Written by the indexing model from the issue text.