Add comment and links explaining Umfpack import

skimage/segmentation/random_walker_segmentation.py

@@ -13,6 +13,12 @@ import warnings
 import numpy as np
 from scipy import sparse, ndimage
 
+# executive summary for next code block: try to import umfpack from
+# scipy, but make sure not to raise a fuss if it fails since it's only
+# needed to speed up a few cases.
+# See discussions at:
+# https://groups.google.com/d/msg/scikit-image/FrM5IGP6wh4/1hp-FtVZmfcJ
+# http://stackoverflow.com/questions/13977970/ignore-exceptions-printed-to-stderr-in-del/13977992?noredirect=1#comment28386412_13977992
 try:
     from scipy.sparse.linalg.dsolve import umfpack
     old_del = umfpack.UmfpackContext.__del__