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 ramp test image.  2.  Threshold using ops and count the
number of foreground pixels.  3.  Convert to IJ1 ImagePlus and threshold
using the ImageProcessor threshold functions 4.  Verify the count
produced by the op is the same as the count produced by ImageJ1.

imglib2-ij was added as a testing scope dependency as it contains the
utilities to convert to IJ1 ImagePlus.
HistogramCreate fixes

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.

HistogramCreate is made an op so that the thresholding methods can find
it using the op service.
Threshold op now passed to map op

Previously a threshold of type T was passed to the map op.  However the
map needs a function 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.