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 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.