FIX: Ensure loaded GIFTI files expose writable data arrays

[effigies]

Fixes #746.

[effigies]

So I dug a bit into our options for finding ways of translating from B64BIN/B64GZ to a bytearray with fewer copies, but at least the out-of-the-box attempts like BytesIO + GzipFile will make copies internally and seem unlikely to save us much.

On the plus side, data arrays will rarely get larger than a point set (up to 163842x3 float64s) or triangle mesh (~330000x3 int32s), which will each be less than 4MB, so one or two unnecessary copies are unlikely to cause an out-of-memory error, and the XML parsing will probably be more expensive.

That said, if anybody knows some clever tricks, I'm happy to hear them.

This is ready for review. Travis failures will be fixed with #749.

[pauldmccarthy]

I tried and failed to fix this issue with indexed_gzip^*, but I'm wrapping the IndexedGzipFile class with an io.BufferedReader which, regardless of what I do in the IndexedGzipFile class, will always return a bytes object :(

However, I personally don't need the data to be writeable - the only reason I was bitten by this is because pyopengl currently (and unnecessarily) requires writeable arrays. Personally, I'd be inclined to just return read-only data - if the caller needs the data to be writeable, they can explicitly take on the burden of creating the copy.

But as you said, these arrays aren't very big most of the time, so copying is unlikely to be a bottleneck.

^*I didn't try very hard, so may have a play with it again some time in the future.

[effigies]

@pauldmccarthy Pretty much all of our image types expose writable data arrays, so I'm inclined to call their being read-only a bug.

[codecov-io]

Codecov Report

Merging #750 into maint/2.4.x will increase coverage by <.01%.
The diff coverage is 91.66%.

@@               Coverage Diff               @@
##           maint/2.4.x     #750      +/-   ##
===============================================
+ Coverage        88.36%   88.37%   +<.01%     
===============================================
  Files              188      188              
  Lines            23998    23986      -12     
  Branches          4258     4256       -2     
===============================================
- Hits             21207    21197      -10     
+ Misses            2103     2100       -3     
- Partials           688      689       +1

Impacted Files	Coverage Δ
nibabel/gifti/parse_gifti_fast.py	84.54% <91.66%> (+0.03%)	⬆️
nibabel/cmdline/parrec2nii.py	32.73% <0%> (ø)	⬆️
...ib/site-packages/nibabel/gifti/parse_gifti_fast.py	84.54% <0%> (+0.03%)	⬆️

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[effigies]

Any further comments/reviews?