Write out coregistration transform

[tsalo]

Summary

QSIPrep doesn't currently write out the transform from dwiref to ACPC.

It would be written out as:

sub-<label>/
    dwi/
        <source_entities>_from-dwiref_to-ACPC_mode-image_xfm.h5

[mattcieslak]

The dwiref is already in acpc. Should we add back the identity transform?

[tsalo]

I think we should write out the dwiref in dwiref space in addition to the ACPC-space one.