ENH: Add ndim to ArrayProxy and DataobjImage

[effigies]

Closes #595.

[coveralls]

Coverage increased (+0.007%) to 91.834% when pulling ce210bd on effigies:enh/arrayproxy_ndim into 7877add on nipy:master.

[codecov-io]

Codecov Report

Merging #674 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #674      +/-   ##
==========================================
+ Coverage   88.86%   88.87%   +<.01%     
==========================================
  Files          93       93              
  Lines       11378    11386       +8     
  Branches     1869     1869              
==========================================
+ Hits        10111    10119       +8     
  Misses        930      930              
  Partials      337      337

Impacted Files	Coverage Δ
nibabel/ecat.py	85.87% <100%> (+0.08%)	⬆️
nibabel/minc1.py	90.06% <100%> (+0.12%)	⬆️
nibabel/arrayproxy.py	100% <100%> (ø)	⬆️
nibabel/parrec.py	94.36% <100%> (+0.02%)	⬆️

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[effigies]

@cancan101 Would you like to test that this provides the interface(s) you were asking for?

[effigies]

@matthew-brett Up for a quick review? This is a pretty small change.

[matthew-brett]

Thanks - yes - it looks fine. The only question is whether it's really worth adding the code for the benefit of going from ndim = dataobj.ndim to ndim = len(dataobj.shape). What do you think?

[effigies]

I agree it's trivial. The advantage of the addition to ArrayProxy is that now ndim exists regardless of how the image is created. It's just a least surprise sort of thing. Passing it though to the image itself is less obviously simplifying, but what #595 originally asked about, so I went ahead and included it.

[matthew-brett]

Then I'm fine with it, if you are ...