Optimize Reference Figures Download by Checking Version

[cbender98]

Please describe your 🚀 feature request.
For the generate_overview function, improve the speed. Currently, the script downloads the reference figures every time it runs, even though this is unnecessary if the latest version of the reference figures is already downloaded.

Describe the solution you'd like
Check the simpa version in the Nextcloud folder and compare it with the version of the already downloaded reference figures. Only download the Nextcloud folder if the versions differ.

Additional context
I’m unsure how to quickly check the versions without downloading all the files.