ENH: Add a class for generating an image histogram.
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