FIX: Reorient fieldmaps to RAS before estimating B-splines

[effigies]

In #339, we ensured that the b-spline coefficients were interpreted in RAS when applied, but failed to ensure that they were calculated in RAS when deriving from pre-existing fieldmaps.

I still need to carefully walk through a workflow directory created fresh with this patch to verify that everything is as it should be.

Fixes #353.

[effigies]

Okay. I had to be a bit more thorough to ensure that the debug maps were generated properly, but this looks good now. With this, the outputs of bs_filter are always in RAS. So I think we have:

1. PEPolar: Orig -> ReorientToLAS -> LAS -> TOPUP -> LAS (affine says RAS) -> TOPUPCoeffReorient -> RAS
2. phasediff/phase1+2: Orig -> Phasediff2Fieldmap -> Orig -> BSplineApprox -> RAS
3. Fieldmap: Orig -> CheckB0Units -> Orig -> BSplineApprox -> RAS
4. SyN: Orig -> SyN -> LPS (affine says RAS) -> DisplacementsField2Fieldmap -> RAS -> BSplineApprox -> RAS

[effigies]

@oesteban Do you have any time to review? I consider this a bug fix and have tested locally and I'm okay merging if tests are green.

[codecov-commenter]

Codecov Report

Patch coverage: 20.00% and project coverage change: -0.26 ⚠️

Comparison is base (5ef62fc) 83.25% compared to head (03b9e8a) 83.00%.

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Additional details and impacted files

@@               Coverage Diff               @@
##           maint/2.4.x     #354      +/-   ##
===============================================
- Coverage        83.25%   83.00%   -0.26%     
===============================================
  Files               25       25              
  Lines             2096     2106      +10     
  Branches           249      251       +2     
===============================================
+ Hits              1745     1748       +3     
- Misses             307      313       +6     
- Partials            44       45       +1     

Impacted Files	Coverage Δ
sdcflows/interfaces/bspline.py	75.19% <20.00%> (-1.86%)	⬇️

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