Make downloads and caching more stable

[teutoburg]

This should be an overview issue about any network-related functionality in ScopeSim:

• Replace astropy.utils.data.download_file with httpx #332
• Improve caching generally
• (Online) tests should use ScopeSim_Data #323
• "Notebooks with IRDB download" test often fails with 403 error from github #254
• At some point remove the scopesim/data/svo directory. Perhaps once speXtra caching is properly functional and ScopeSim_Data includes those files.
• Make ScopeSim_Data a dependency of 'ScopeSim the application', perhaps after we split off scopesim_core?
• Optionally, set-up a test infrastructure that blocks use of the internet once the packages are installed.
• Optionally, create guix packages for ScopeSim because those cannot use the internet while testing.

Feel free to add to this list...