AbstractComputeThresholdHistogram: fix createOutput

It was broken because previously, there was no way to obtain the type T
of a Histogram1d at runtime. But as of imglib2 2.0.0-beta-27, the
Histogram1d has a firstDataValue() method that returns the first element
of the data that was counted.
Merge pull request #59 from imagej/threshold-output-creation

Threshold output creation