ENH: volume direction vectors

[bennyrowland]

It would be nice to be able to access the individual direction vectors of an image volume, normalised, to make it easy to construct an aligned coordinate system for slicing other volumes, for example. It would be even better to be able to also ask for the axial/sagittal/coronal directions (or closest approximations) so that such slices can be constructed easily without reference to the original planes of the image.