PKG: Rename _ssim to _structural_similarity.

skimage/measure/__init__.py

@@ -1,3 +1,4 @@
 from .find_contours import find_contours
 from ._regionprops import regionprops
-from ._ssim import ssim
+from .find_contours import find_contours
+from ._structural_similarity import ssim

skimage/measure/tests/test_structural_similarity.py

@@ -1,7 +1,8 @@
 import numpy as np
 from numpy.testing import assert_equal
 
-from skimage.measure._ssim import structural_similarity as ssim, _as_windows
+from skimage.measure._structural_similarity import \
+     structural_similarity as ssim, _as_windows
 import scipy.optimize as opt
 
 def test_ssim_patch_range():