ENH: Add exists=True for out_label_fusion output file

The required output file must exist to be a valid completion of AntsJointFusion, so
add this check to the file.

Author: hjmjohnson

nipype/interfaces/ants/segmentation.py

@@ -940,7 +940,7 @@ class AntsJointFusionInputSpec(ANTSCommandInputSpec):
 
 
 class AntsJointFusionOutputSpec(TraitedSpec):
-    out_label_fusion = File()
+    out_label_fusion = File(exists=True)
     out_intensity_fusion_name_format = traits.Str()
     out_label_post_prob_name_format = traits.Str()
     out_atlas_voting_weight_name_format = traits.Str()