Revert "changing in_files in RealignInputSpec"

This reverts commit f196238.

Author: effigies

nipype/interfaces/spm/preprocess.py

@@ -125,7 +125,8 @@ def _list_outputs(self):
 
 class RealignInputSpec(SPMCommandInputSpec):
     in_files = InputMultiPath(
-        ImageFileSPM(exists=True),
+        traits.Either(traits.List(ImageFileSPM(exists=True)),
+                      ImageFileSPM(exists=True)),
         field='data',
         mandatory=True,
         copyfile=True,