Support to load single volumes from 4D images

[mih]

Dave Flitney requested a leaner way to get access to parts of a 4D image (NIfTI in his case), i.e. the ability to get data of a single volume or timeseries. He sent the following sketch::

class FSLNiftiImage(nb.Nifti1Image):
#    def __init__(self):
#       super(FSLNiftiImage, self).__init__()

    @memoized
        def get_fileobj(self):
        fileobj = allopen(self.get_filename())
        return fileobj

    def get_volume(self, v):
        dtype = self.get_data_dtype()
        s = self.get_header().get_data_shape()
        shape = (s[0], s[1], s[2])
        offset = self.get_header().get_data_offset() + (v * np.prod(shape))
        data = array_from_file(shape, dtype, self.get_fileobj(), offset)
        return data

Usage:

image = nb.load(fname)
image.__class__ = FSLNiftiImage
v27 = image.get_volume(27)

[effigies]

Closed by #258.