Rename AutoThresholdMethod to ComputeThreshold

We want to differentiate between the layer that _computes_ thresholds,
versus the layer that _applies_ them by iterating over an image.
GlobalThresholder: remove misleading comment
ApplyThreshold: add javadoc TODO reminder
ApplyThreshold: use Comparable<? super T>

The Comparable interface is not recursive... rather, it should be
allowed to use _any_ Comparable implementation that is as general as,
or more general than, the type T. This approach (Comparable<? super T>)
is the typical pattern in the Java standard library.