Replace `mcflirt` with `3dVolreg`

[chrisgorgo]

• Consistent with MRIQC
• Faster than MCFLIRT
• More accurate than MCFLIRT (https://www.ncbi.nlm.nih.gov/pubmed/16099178)

[oesteban]

This would be nice to finish up, but we need to figure out nipy/nitransforms#45 first.

[jbh1091]

With nipy/nitransforms#45 closed as completed, is this now doable?

I see an example implementation in fmriprep-rodents (nipreps/fmriprep-rodents#15) but it looks like that workflow is not actually used anywhere.

[effigies]

Yes, I think it should be.