Compress and cleanup downloaded datasets

[fepegar]

Resolves #250.

Description
After downloading a dataset, remove the zip file and compress all images using gunzip.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #531 (da71472) into master (01924a8) will increase coverage by 1.03%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
==========================================
+ Coverage   90.47%   91.51%   +1.03%     
==========================================
  Files         132      124       -8     
  Lines        6670     6491     -179     
==========================================
- Hits         6035     5940      -95     
+ Misses        635      551      -84     

Impacted Files	Coverage Δ
torchio/utils.py	89.02% <25.00%> (-2.28%)	⬇️
torchio/transforms/transform.py	84.58% <0.00%> (-2.53%)	⬇️
torchio/datasets/episurg.py	27.27% <0.00%> (-1.85%)	⬇️
torchio/datasets/ixi.py	39.04% <0.00%> (-1.70%)	⬇️
torchio/cli/apply_transform.py	76.92% <0.00%> (-0.58%)	⬇️
torchio/data/queue.py	86.00% <0.00%> (-0.28%)	⬇️
torchio/transforms/preprocessing/spatial/pad.py	89.47% <0.00%> (-0.27%)	⬇️
torchio/data/subject.py	96.52% <0.00%> (-0.10%)	⬇️
torchio/data/image.py	88.81% <0.00%> (-0.08%)	⬇️
... and 15 more

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