ENH: Create gzip header deterministically by default

[effigies]

As discussed in #1023, we currently allow gzip.GzipFile to set the gzip header as it will, including filename and mtime fields. This means that files with identical unzipped contents and compression levels have different gzipped representations, which is not ideal for verifying sources of non-determinism in a complex workflow.

This PR attempts to balance sensible default behaviors with user control. It changes the default mtime from time.time() to 0, while allowing users to set the mtime as an argument to Opener().

It removes the behavior of setting the original filename in the gzip header. In a medical imaging context, the original filenames may include PHI (e.g., subject name, scan date, etc) that will follow the file through renames unless the file is rewritten entirely. A flag to preserve the old behavior does not seem worth the effort.

Fixes #1023.

[effigies]

@yarikoptic @ltetrel Would either of you care to review?

[codecov]

Codecov Report

Merging #1024 (ff5efe4) into master (dd3fae3) will decrease coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1024      +/-   ##
==========================================
- Coverage   92.26%   92.10%   -0.16%     
==========================================
  Files         100      100              
  Lines       12201    12205       +4     
  Branches     2134     2136       +2     
==========================================
- Hits        11257    11242      -15     
- Misses        616      630      +14     
- Partials      328      333       +5     

Impacted Files	Coverage Δ
nibabel/openers.py	93.16% <100.00%> (-2.42%)	⬇️
nibabel/pydicom_compat.py	61.53% <0.00%> (-30.77%)	⬇️
nibabel/volumeutils.py	92.22% <0.00%> (-1.30%)	⬇️
nibabel/viewers.py	95.49% <0.00%> (-0.65%)	⬇️

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[yarikoptic]

Looks great, thank you @effigies !!!

[effigies]

Thanks for the review.

[kousu]

This is great! Thank you for being proactive about squashing the indeterminism/data leakage.