Add global threshold test class

This class is for testing global threshold methods.  The initial commit has a test for Otsu threshold.  In the future tests can be added for other global threshold methods.  The Otsu test is structured as follows:

1.  Create a simple ramp test image.
2.  "Offline" segment this image using a known implementation (in this case the ImageJ1 implementation of Otsu)
3.  Count the number of foreground pixels.
4.  Verify the count produced by the op is the same as the count produced by ImageJ1.
Call countData

In 'HistogramCreate.run' a histogram class is created but the histogram data does not seem to be generated.  It seems a call to 'countData' is needed to generate the histogram of the input.
Threshold op now passed to map op

Previously a threshold of type T was passed to the map op.  However the map needs a fucntion as the third input parameter.
Threshold converted from bin number to gray level

In Otsu.java getThreshold, the threshold is calculated as a bin number.  Added code to convert it to a gray level.