Proposed fix for Unit definition

[SergejMuhic]

Fixes: Fixes #9

#22

Changes: Created a new Unit element in units of type=ifc:IfcUnit

Attention:

1. I was a bit confused with the existing definition and could not narrow down the intention. It could be that the proposed fix does not address the requirement.
2. Verify by compiling the xsd

[pjanck]

Right, enabling this in IDE recognises the schema:

Now, the type https://standards.buildingsmart.org/IFC/RELEASE/IFC4/Add2TC1:IfcUnit is not declared, which is peculiar. Investigating further.

[pjanck]

As it stands today, the IFC schema referenced has a problem and is not serializable as well. Investigating further with @SergejMuhic and @apinzenoehler .