Add zarr to TIFF series converter

[msayr]

• Add emalign/io/zarr_to_tiff.py which opens a zarr dataset with tensorstore, reads each Z slice and writes it as an uncompressed TIFF using tifffile.imwrite(compression=None).
• Writes one .tiff file per slice (filename prefix and zero-padding width configurable via --prefix and --digits).
• Adds options for slice range (--slice-range), concurrency (-c/--cores), and --overwrite to replace existing files, and includes input validation for slice ranges.
• Uses tqdm for progress reporting and ThreadPoolExecutor to parallelize writes when requested.