SDC-SyN: Images with oblique xforms

[oesteban]

As pointed out by @mattcieslak.

This issue can be closed with:

• Replace the xforms with the corresponding matrix aligned with the imaging axes
• Running SDC-SyN
• Setting the original xforms on the displacements field (checking whether this produces the transform along the oblique axis).

[mattcieslak]

Also should ping @cookpa on this one

[oesteban]

After pushing another RC of 1.4.0, I think I'll be ready to work on this.