FIX/TEST: Gunzip cleanup and test

[effigies]

Following Neurostars #1623, I checked on the nipype Gzip implementation. Apparently I misremembered, because I found Gunzip instead. Anyway, here are some cleanups and a doctest.

Changes proposed in this pull request

• FIX: _gen_output_file_name had a bad test for .gz extension
• ENH: Use shutil.copyfileobj to ensure more memory-friendly buffering
• TEST: Add doctest to demonstrate usage and verify filename change

[codecov-io]

Codecov Report

Merging #2564 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2564      +/-   ##
==========================================
+ Coverage   67.13%   67.13%   +<.01%     
==========================================
  Files         333      333              
  Lines       42542    42543       +1     
  Branches     5265     5265              
==========================================
+ Hits        28560    28562       +2     
+ Misses      13280    13279       -1     
  Partials      702      702

Flag	Coverage Δ
#smoketests	50.71% <100%> (ø)	⬆️
#unittests	64.47% <75%> (ø)	⬆️

Impacted Files	Coverage Δ
nipype/algorithms/misc.py	45.97% <100%> (+0.17%)	⬆️

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