Allow users to ignore precomputed volume types

[tsalo]

Summary

As multiple folks have noted, the precalculated CBF maps coming off the scanner are often pretty bad, but ASLPrep will use them to generate the ASL reference image when they're available. We should probably give users the ability to ignore these. Something like --ignore-volume-types <cbf|deltam|m0scan>?

[tsalo]

Perhaps just adding them as options for --ignore would be sufficient.