ENH: Added valley emphasis algorithm to OtsuMultipleThresholds

The valley emphasis algorithm is from H.F. Ng, "Automatic thresholding
for defect detection", Pattern Recognition Letters, (27): 1644-1649,
2006.  It is particularly effective when the object to be thresholded
is small.  Added a test to demonstrate the difference between turning
it off (itkOtsuMultipleThresholdsImageFilterTest3) and on
(itkOtsuMultipleThresholdsImageFilterTest4).

Most of the code in this patch was contributed by Glen Brooksby, GE
Global Research.

Change-Id: I748738644ba00d2ea5d5254e0e080aaeecc00f13
Merge topic 'OtsuValleyEmphasis'

fb21473 ENH: Added valley emphasis algorithm to OtsuMultipleThresholds