InteractiveImageCommand: fix javadoc

It had an obsolete reference to ij-commands-display, and also
did not mention the Threshold command as a second example.
ImageCalculator: use CalculatorOp as @Parameter

This is now possible thanks to scijava-common 1.6.0's support for
singleton plugin instances being automatically handed to ObjectService.

So ImageJ2's ObjectWidget can query the ObjectService for objects which
implement CalculatorOp, and automatically populate the dropdown list!
Threshold: use ThresholdMethod as @Parameter

This is now possible thanks to scijava-common 1.6.0's support for
singleton plugin instances being automatically handed to ObjectService.

So ImageJ2's ObjectWidget can query the ObjectService for objects which
implement ThresholdMethod, and automatically populate the dropdown list!
Merge branch 'singleton-objects'

This uses SciJava Common 1.6.x's new capability to retrieve singleton
plugin instances from the object service. We can now use such plugins
as @Parameters directly, without needing to jump through hoops to
convert between the singletons and string representations thereof.