Interface to convert  3dvolreg output output to a set of ITK affine matrices

Some references:

[Code to convert AFNI motion parameters to a numpy affine matrix](https://github.com/nipy/nipype/blob/master/nipype/algorithms/rapidart.py#L42)

[Discussion about peculiarities of ITK affine matrix file format](https://mail.python.org/pipermail/neuroimaging/2015-July/000145.html)

@matthew-brett let us know if you prefer this functionality to end up in nibabel.