Ensure download directory for scopesim is writable
Currently the scopesim files are downloaded to the current working directory. As a result, it is not possible to run notebooks that are in a non-writable directory. E.g. it is not possible to create a symlink to the notebooks on the practicum jupyter hub.
It should be possible to run these notebooks, so maybe the directory should be set to e.g. /tmp
or so if the current working directory is not writable.