Time-slicing spatial images

[effigies]

RE: #271 and #404

So, I recognize the issues involved with spatial slicing of SpatialImages, but a pretty handy thing to be able to do would be: img[...,slicer].orthoview()

Almost always I'm loading the file on the spot, so really it's: nib.load(fname)[...,slicer].orthoview()

Being a one-liner from filename to display is the real beauty of this, but the options are:

x = nib.load(fname)
Nifti1Image(x.dataobj[...,slicer], x.affine, x.header).orthoview()
# or
nib.viewers.OrthoSlicer3D(x.dataobj[...,slicer], x.affine)

Would it be acceptable to modify SpatialImage.__getitem__ to accept a time slice and return a SpatialImage, but reject spatial slices?

[effigies]

I suppose an acceptable alternative that doesn't involve mucking with __getitem__ would be:

nib.load(fname).orthoview(timeslice)

The only annoyance I see there is the need to manually define the slice, instead of getting the start:stop:step syntax.