Since the Importer is a class, I have mixed-in the Cleanup module. This is idiomatoc Ruby, but less typical for SketchUp extensions. The file is [`included`](https://github.com/jimfoltz/sketchup-stl/blob/feat/extract-cleanup-methods/src/sketchup-stl/importer.rb#L16) in the importer class, so when the method is called, it is just `cleanup_geometry(entities)`