Ensure data are in LAS+ early on in pipeline

[tsalo]

Summary

@mattcieslak mentioned today that the preps use RAS+, while FSL typically uses LAS+, and FSL isn't great at checking that orientations match. It's possible that prep preprocessing could force the orientation is RAS+, but the raw data might still be in LAS+ and any FSL-specific steps that use both images could ignore the orientation mismatch.

ASLPrep might already have a step that does this. I'll need to dig into the code to make sure.

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