feat: Add N4 parameters to init_syn_preprocessing_wf

[effigies]

These are changes that are patched in to the epi_reference_wf in SyN-SDC in https://github.com/nipreps/fmriprep-rodents/pull/55/files.

The expected diff would be:

             syn_preprocessing_wf = init_syn_preprocessing_wf(
                 omp_nthreads=config.nipype.omp_nthreads,
                 debug=config.execution.debug is True,
                 auto_bold_nss=True,
                 t1w_inversion=False,
+                n4_iterations=[50] * 4,
+                n4_shrink_factor=1,
+                calculate_n4_spline_grid=True,
                 name=f"syn_preprocessing_{estimator.bids_id}",
             )
             syn_preprocessing_wf.inputs.inputnode.in_epis = sources
             syn_preprocessing_wf.inputs.inputnode.in_meta = source_meta
-
-            #  The default N4 shrink factor (4) appears to artificially blur values across
-            #  anisotropic voxels. Shrink factors are intended to speed up calculation
-            #  but in most cases, the extra calculation time appears to be minimal.
-            syn_preprocessing_wf.inputs.epi_reference_wf.n4_avgs.shrink_factor = 1
-            #  Similarly, the use of an asymmetric bspline grid improves performance
-            #  in anisotropic voxels, so set INU bspline grid based on voxel size
-            syn_preprocessing_wf.inputs.epi_reference_wf.n4_avgs.args = _bspline_grid(sources[0])
-            # The number of N4 iterations are also reduced.
-            syn_preprocessing_wf.inputs.epi_reference_wf.n4_avgs.n_iterations = [50] * 4

cc @eilidhmacnicol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.15%. Comparing base (b588f75) to head (1d90e1d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #503   +/-   ##
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  Coverage   84.15%   84.15%           
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  Files          30       30           
  Lines        2872     2872           
  Branches      382      382           
=======================================
  Hits         2417     2417           
  Misses        381      381           
  Partials       74       74           

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