Fieldmap integration

[oesteban]

Currently, we just have a placeholder.

My idea is to integrate this in the LOGO splitter:

• We already have a copy of the fieldmap (I'd say the coefficients of its B-Spline representation) in the DWI object.
• We already have a list of realigning affines (which are identity in the first step).
• We then project the fieldmap coefficients into the left-out gradient through the corresponding affine.
• We interpolate the fieldmap at every voxel from the coefficients.
• We resample the left-out gradient with the projected fieldmap (I think nitransforms should be able to do this already).
• We return a left-out gradient after unwarping!

Therefore, this simplifies something that otherwise would be really convoluted: it's not just that the fieldmap is included in the process, it is that we will be able to account for susceptibility-distortions in the moved space of each volume (which makes the wapring a little bit different for each orientation)