Map out and refactor the code base to make sure the toolkit is sustainable in longer term

#### Description:

Following topics need to be mapped out and confirmed:
- file structure, core mechanics of the adapter
- libraries used to parse the IFC (currently [xbim](https://github.com/xBimTeam/XbimEssentials))
- need for discipline-specific pull, similar to Revit_Toolkit?

I will start working on the above right after #1, to be expanded into a range of more granular issues. @al-fisher @FraserGreenroyd @alelom please feel free to comment or add other items you think should be covered on such an early stage. Thanks!