Step through interactive runner for pipeline

[kiyo-masui]

It would be great if caput.pipline.Manager could be stepped through interactively. One would have to break apart the run() method into a series of calls to a new method next(). The new method could return any outputs for the user to inspect (while still queueing them up for further processing).

In addition, it would be nice if there was a way to visualize the current state of the pipeline. Just a fancy print that gives the lengths of the queues, etc.

[jrs65]

One additional idea could be putting a breakpoint: True into the YAML file, such that it drops into an interactive mode. You could probably even force it to drop into pdb.