do not hook _nthreads_update to inputs.num_threads changes for afni interfaces that do not have the input

Author: oesteban

nipype/interfaces/afni/base.py

@@ -201,7 +201,9 @@ def set_default_output_type(cls, outputtype):
     def __init__(self, **inputs):
         super(AFNICommand, self).__init__(**inputs)
         self.inputs.on_trait_change(self._output_update, 'outputtype')
-        self.inputs.on_trait_change(self._nthreads_update, 'num_threads')
+
+        if hasattr(self.inputs, 'num_threads'):
+            self.inputs.on_trait_change(self._nthreads_update, 'num_threads')
 
         if self._outputtype is None:
             self._outputtype = Info.outputtype()