[WIP] ENH: Generate conformation transforms

[effigies]

Insomnia led to playing around with this. Includes an assert to verify that the transform actually does get you from one affine to another. Tested on ds031.

Looks like mri_robust_template only outputs LTA transforms.

Assuming this is worthwhile, I'd propose the following derivative structure:

derivatives/
    fmriprep/
        anat/
            transforms/
                <orig_T1w_name>.mat

Related: #624

[effigies]

Closed in favor of #726.