References covid19/pdyn2-stack#353: move gus code to stack
The files that are very specific to the gus model are moved to the private repo.
We do this, because:
- we don't want to put internal (rather messy) to the public, especially since we are not allowed to share the input files
- we want to make obvious which parts of structure creations we need to redo (we hope for an official, public way of building the structure)