Feat/progress

[PGijsbers]

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• Adds a progress bar to MinIO downloads
• When download_all_files=True, only download the files which do not yet exist in cache. Overwriting is no longer necessary, as it was a crutch that was built before force_refresh_cache was added to the get_dataset method. This means we now can also avoid re-downloading image datasets over and over again.

WIP:

• [ ] add/update test for cache behavior
• [ ] update release notes