Adding a butler argument to the constructor signatures for CharacterizeImageTask, CalibrateTask, and ProcessCcdTask makes these tasks difficult to use without a butler.

The fix is to make the butler argument optional (with a default of None), while adding another argument that allows a fully-constructed reference object loader to be provided directly instead.

This is closely related to DM-6597, which has the opposite problem: pipe_drivers' SingleFrameDriverTask doesn't take a butler argument, but it needs to in order to provide one to ProcessCcdTask.

I have a fix for this just about ready, but I'd like to add some unit tests that verify we can run all of these tasks both from the command-line and directly before calling it complete.