Allow nested models

[bloer]

For very complex geometries, it would be useful to manage some of the subassemblies as standalone entities, then reference them for the overall model. .

The extreme extension of this would be to have an entire database of Components, Assemblies, and Emission Specs, and assemble models from them. In order to preserver model versioning, each entry would need to keep track of versions, and we'd probably end up with a huge number of artifacts. And import/export would get really complicated. But being able to share common sub-assemblies across models would be very valuable.