Fix possum test.

FSLOUTTYPE must only default to .nii for me.

Author: tsalo

nipype/interfaces/fsl/possum.py

@@ -80,7 +80,7 @@ class B0Calc(FSLCommand):
     >>> b0calc.inputs.in_file = 'tissue+air_map.nii'
     >>> b0calc.inputs.z_b0 = 3.0
     >>> b0calc.cmdline  # doctest: +IGNORE_UNICODE
-    'b0calc -i tissue+air_map.nii -o tissue+air_map_b0field.nii --b0=3.00'
+    'b0calc -i tissue+air_map.nii -o tissue+air_map_b0field.nii.gz --b0=3.00'
 
     """