Bug fix: nbins can be >256 in threshold_isodata

The threshold_isodata function created an arange of values up to the
number of bins, but gave it type np.uint8, limiting the number of bins
to 256.

Fixes #1085.
Fix incorrect test function signature
Merge pull request #1086 from jni/thresh-iso

Bug fix: nbins can be >256 in threshold_isodata