PipelineStep behaviour/option consistency: "copy all other inputs to output" or not

[AlexDBlack]

Some pipeline steps keep all other inputs (other than the ones being processed), some don't, and some provide the option (with default do/don't)..

We need to decide the criteria for keeping vs. not the inputs (and then align the behaviour of all pipeline steps:

• When should they have this option?
• Should it be hardcoded or configurable?
• If configurable, what should the default be?