Fix isodata-thresholding according to Zachary Pincus
Return to old function name threshold_isodata
Account for case when image only contains one unique value
Fix test cases which are now wrong due to previous binning error
Improve test coverage
Fix return value description
Test missed line for coverage
Merge pull request #1090 from ahojnnes/isodata-fixes

Fix isodata-thresholding according to Zachary Pincus