Use Ants instead of mri_coreg

[bendhouseart]

Ants is a bit faster than mri coreg and there's already a workflow for it in

see niworkflows RobuestMNINormalization requires ants registration file

https://github.com/ANTsX/ANTs
https://github.com/nipreps/mriqc/blob/add_petqc/mriqc/workflows/pet/base.py

[bendhouseart]

Check to see how it's used to co-register other images in niworkflows:
https://github.com/nipreps/niworkflows/tree/master/niworkflows/data

And Martin's PET version:
nipreps/niworkflows#950

[bendhouseart]

Take a look at the visualization workflow to see how/if there's an output figure, eg work report.