FIX: Detect IndexedGzipFile as compressed file type

[pauldmccarthy]

This PR updates nibabel.volumeutils so that it recognises IndexedGzipFile objects as compressed files, and does not attempt to memory-map them.

Background at pauldmccarthy/indexed_gzip#38.

[codecov]

Codecov Report

Merging #925 into maint/3.1.x will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           maint/3.1.x     #925   +/-   ##
============================================
  Coverage        91.77%   91.77%           
============================================
  Files               97       97           
  Lines            12359    12360    +1     
  Branches          2177     2177           
============================================
+ Hits             11343    11344    +1     
  Misses             681      681           
  Partials           335      335           

Impacted Files	Coverage Δ
nibabel/openers.py	95.09% <100.00%> (+0.04%)	⬆️
nibabel/volumeutils.py	84.52% <100.00%> (ø)

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[effigies]

@pauldmccarthy Thanks for this. It looks safe and bug-fixy. Would you mind rebasing on maint/3.1.x?

[pauldmccarthy]

hey @effigies, no problem; I'm not sure if you meant to rebase against maint/3.1.x, as that would result in a PR containing all of the differences between master and maint/3.1.x. What I did instead was re-create the feature branch off maint/3.1.x, and then cherry pick the commit for this PR onto it. Let me know if that's what you wanted!

[effigies]

Yup, this is exactly what I meant. Thanks!