Output SDC transformations as voxel displacement maps

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The simplest representation of a 3D warp is a voxel displacement map. This can be represented as a NIfTI-1 file of shape IxJxKx3.

Part of #2207.

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Currently outputting fieldmaps in Hz, which is probably fine.

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Correction: We need to emit an affine transformation from each BOLD to its corresponding fmap.