reworked ThresholdTwoLevels (3rd try)
change output to uint32 everywhere
fixes #763
Merge branch 'pr-thresholding-rewrite3' into pr-thresholding-rewrite4

Conflicts:
	ilastik/applets/thresholdTwoLevels/opThresholdTwoLevels.py
Merge pull request #848 from burgerdev/pr-thresholding-rewrite4

rewrite of ThresholdTwoLevels (rebased to current master)