try to fix dartel_template format bug

Author: bpinsard

nipype/interfaces/spm/preprocess.py

@@ -1341,7 +1341,7 @@ def _format_arg(self, opt, spec, val):
             if val == 'low':
                 return {'normlow': []}
         elif opt in ['dartel_template']:
-            return scans_for_fnames(val, keep4d=True)
+            return np.array([val], dtype=object)
         elif opt in ['deformation_field']:
             return super(VBMSegment, self)._format_arg(opt, spec, [int(val[0]), int(val[1])])
         else: