Add ability to ingress sMRIPrep derivatives with `--datasets`/`--derivatives` argument

[tsalo]

Summary

We don't plan to directly incorporate sMRIPrep into QSIPrep, but I think it would be great to allow users to reuse existing sMRIPrep derivatives. We can use the preprocessed T1w/T2w from sMRIPrep as the "anat" space, from which the "ACPC" space can be generated.

[tsalo]

With sMRIPrep, we should get transforms from anat to any number of output spaces. We would probably just need to take the ACPC-to-anat transform and combine it with the anat-to-template transforms from sMRIPrep to get the ACPC-to-template transforms we want for QSIPrep/QSIRecon.