FIX: Make ants.LaplacianThickness output_image a string, not file

Closes gh-3390

Author: effigies

Diff for: nipype/interfaces/ants/segmentation.py

@@ -268,7 +268,7 @@ class LaplacianThicknessInputSpec(ANTSCommandInputSpec):
         desc="gray matter segmentation image",
         position=2,
     )
-    output_image = File(
+    output_image = traits.Str(
         desc="name of output file",
         argstr="%s",
         position=3,

Diff for: nipype/interfaces/ants/tests/test_auto_LaplacianThickness.py

@@ -36,7 +36,6 @@ def test_LaplacianThickness_inputs():
         ),
         output_image=dict(
             argstr="%s",
-            extensions=None,
             hash_files=False,
             keep_extension=True,
             name_source=["input_wm"],