chore: cosmetic changes & deduplicate _pop definition in ants

cc/ https://github.com/nipreps/niworkflows/pull/408/files#r330252779

Author: oesteban

niworkflows/anat/ants.py

@@ -15,7 +15,7 @@
 from nipype.interfaces.fsl.maths import ApplyMask
 from nipype.interfaces.ants import N4BiasFieldCorrection, Atropos, MultiplyImages
 
-from ..utils.misc import get_template_specs
+from ..utils.misc import get_template_specs, select_first as _pop
 
 # niworkflows
 from ..interfaces.ants import (
@@ -897,12 +897,6 @@ def init_n4_only_wf(
     return wf
 
 
-def _pop(in_files):
-    if isinstance(in_files, (list, tuple)):
-        return in_files[0]
-    return in_files
-
-
 def _select_labels(in_segm, labels):
     from os import getcwd
     import numpy as np

niworkflows/utils/misc.py

@@ -215,10 +215,13 @@ def splitext(fname):
 def select_first(in_files):
     """
     Select the first file from a list of filenames.
+
     Used to grab the first echo's file when processing
     multi-echo data through workflows that only accept
     a single file.
 
+    Examples
+    --------
     >>> select_first('some/file.nii.gz')
     'some/file.nii.gz'
     >>> select_first(['some/file1.nii.gz', 'some/file2.nii.gz'])
@@ -227,8 +230,7 @@ def select_first(in_files):
     """
     if isinstance(in_files, (list, tuple)):
         return in_files[0]
-    else:
-        return in_files
+    return in_files
 
 
 def _copy_any(src, dst):