Using Parts - correct properties in the IFC

I did some testing on exporting Parts to IFC and had multiple problems with the properties.

A Part:
- has a Common Property Value but it is not related to the Host
- IsExternal
- has Common Properties but no value - so also not related to the Host
- Fire Rating
- Load Bearing
- doesn't have the following Properties but the Host has
- Type Name
- Uniformat Classification
- Reference (original Family and Type in Revit)
- Thickness (Revit knows if the geometry is changed)
- has it's own Parameters based on beiing a Part (this makes sense)
- Name
- Type
- has a different IFC entity from the host? > only mapping by Category?

There are real live occasions where you would like to export Parts to IFC.
But at the moment it doesn't have the basic quality of the properties that is needed.

I would love to see these things fixed in future releases of the exporter.
So users do not need to build their own buggy solutions.
(there is not always a direct link from a part to 1 original host)

Here are some simple test files.
[190708_Parts_test_file.zip](https://github.com/Autodesk/revit-ifc/files/3367849/190708_Parts_test_file.zip)

Teun de Vries.

Start with the attached 190708_Parts_test_file.zip and reproduce the IFC export for Parts. Compare the exported Part and host entities and their properties against the listed expectations, including IsExternal, Fire Rating, Load Bearing, Type Name, Uniformat Classification, Reference, and Thickness. Done means the relevant properties and host relationships export correctly without requiring custom solutions.

Written by the indexing model from the issue text.