The new vtkImageHistogram class is a multi-threaded filter for
computing an image histogram that differs from vtkImageAccumulate
in that the output image is a histogram plot that can be directly
displayed to the user.  It is also possible to get the histogram
as a vtkIdTypeArray via the GetHistogram() method.

The subclass vtkImageHistogramStatistics computes image statistics
from the generated histogram.  In addition to providing mean, median,
and standard deviation, this class can be used to compute optimal
window/level values for image viewing based on histogram percentiles.

Patch Set 3: Add SetActiveComponent method.

Change-Id: I31c12428b54471c1b6a6e874b219dfacb27282d0