OpThresholdTwoLevels: If the user adds a file that doesn't have enough channels for the current Channel setting, raise a DatasetConstraintError.
precomputed features: fix axistags and dtype bugs
object feature examples
rename 0.6 to 1.0 in developer docs
reference the example features
mention .ilastikrc file
Merge branch 'master' into issue-910

Conflicts:
	ilastik/applets/thresholdTwoLevels/opThresholdTwoLevels.py