Fix the output files of sct_smooth_spinalcord (smooth.nii.gz, straight_ref.nii.gz)

[joshuacwnewton]

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Description

Two small fixes to address the observations from the SCT Course tutorial PR:

1. Output the smoothed spinal cord as a compressed NIfTI file, rather than an uncompressed file.
2. Output the straight_ref.nii.gz file (which is needed for reusing cached results, but wasn't copied over from the temp directory).

An aside...

I'm a little suspicious about this caching behavior... the cache logic is repeated across multiple scripts (smooth_spinalcord, label_vertebrae, register_to_template), but there are subtle changes each time... I wonder if it could be handled by sct_straighten_spinalcord itself instead. That being said, I'd rather keep this change small for now.

Linked issues

Fixes #3503.