Minimal eddy-current correction

With FSL Eddy disabled, there is only the rudimentary motion-correction applied. Will need to improve this if working with e.g. high b-value datasets, or just to improve distortions.

Start by locating the Snakebids app's DWI-processing entry point and the current motion-correction path used when FSL Eddy is disabled. Review how high b-value datasets and distortion correction are currently handled, then define a measurable correction target and validation dataset before changing behavior. Done should mean improved eddy-current correction and reduced distortions for the stated use cases.

Written by the indexing model from the issue text.