Revert "simplify in_file"

This reverts commit f1be287.

Author: bpinsard

nipype/interfaces/spm/utils.py

@@ -211,7 +211,7 @@ def _list_outputs(self):
 
 class ApplyInverseDeformationInput(SPMCommandInputSpec):
     in_files = InputMultiPath(
-        File(exists=True),
+        traits.Either(traits.List(File(exists=True)),File(exists=True)),
         field='fnames',
         mandatory=True,
         desc='Files on which deformation is applied')