Improvements to python-ifc

Dear all :)

I needed to browse through the IFC schemes specifications for my uses and stumbled upon python-ifc which was nearly enough for my uses and simple enough to start using it and adding what I needed.

I then fixed some problems and added features to the initial parser to make it accurately detect the types and entities of the IFC2x3 and IFC4 schemes. The __main__ part now includes a test which checks it is still ok.

I also added the attributes of subtypes and supertypes for each entity, which I am using to make graph visualizations of IFC entities. I can share the code that does it.

Are you still maintaining it ? I saw you just pushed an LGPL license on it 3 days ago. If you lost interest in this little tool I can maintain it if it helps: I think it can have an (even pedagogical) interest.

Hope this pull request helps, feel free to ask for more information of what I implemented !

Guillaume