Regenerate ASL reference image using CBF for final version

[tsalo]

Summary

The initial ASL reference image is created from M0 scans when available, or control images when M0 scans are not available. However, CBF will have a better gray matter-white matter contrast, so, once we have CBF data, it should be a superior reference image for the T1w registration.

Additional details

Thanks to @caugolm for the suggestion.

Next steps

1. Use whatever volume type is available for the initial reference image (as we do now).
2. Once CBF is calculated, use that for the new (final?) reference image.

[tsalo]

I think I'll need to move the CBF workflow into init_asl_fit_wf, so that the coreg aslref can be created fro native-space CBF data instead of the ASL data.