Enable ``to_nifti()`` to generate a split version of the series with motion encoded in q-form (or s-form)

[oesteban]

Instead of resampling, it would be very effective to check for head motion in viewers compliant with NIfTI, as no resampling would be involved if we just update the q-form by the inverse transform of the volume.

[oesteban]

Curious about @effigies' opinion when he's back :)