Add uncertainty / softmax / logit compression

[Karol-G]

This conversion script adds uncertainty / softmax / logit compression to reduce disk space consumption when saving these files. It supports three modes: 8-Bit, 16-Bit and 32-Bit compression. Even for the 8-Bit compression there is probably no difference or loss in performance measurable for downstream-tasks.

The relevant functions are: save_compressed_all: Compresses all Nifti files in a directory. save_compressed: Compresses a single Nifti file. compress_and_save: Compresses an image and saves it. load_compressed: Loads a compressed image in Zarr format, decompresses it and returns the decompressed image.

Best, Karol