DataobjImage doesn't proxy ndim

[cancan101]

DataobjImage currently proxies shape but not ndim even though various docs refer to ndim(s):

• http://nipy.org/nibabel/reference/nibabel.nifti1.html#nibabel.nifti1.Nifti1Header.set_data_shape
• http://nipy.org/nibabel/reference/nibabel.nifti2.html#nibabel.nifti2.Nifti2Header.set_data_shape

Might be useful to proxy ndim through.

[effigies]

Just to be clear, ndims is used in the context of those docstrings just as a stand-in for len(shape), to explain the behavior of the methods.

That said, if people have a use-case for it, it doesn't seem unreasonable to add a property:

@property
def ndim(self):
    return len(self.shape)

Images that explicitly represent this value in the header could override the method to be a little less circuitous.

[matthew-brett]

I'd be OK with adding ndim to the proxy interface. Would need that tested in the image API tests for all the various arrayproxies. @cancan101 - would you consider doing a pull request?