ENH: prevent incorrect calculations when volumes don't match

As pointered out by Yanling Liu on the slicer-users list [1] the
statistics values will be incorrect if the pixels do not match
in the grayscale and label volumes. Now we detect this and
generate a dialog box to prevent miscalculations.

Also minor style adjustments with trailing whitespace.

[1] http://massmail.spl.harvard.edu/public-archives/slicer-users/2013/006487.html

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21940 3bd1e089-480b-0410-8dfb-8563597acbee