add filter to combine pipeline results into a single multi-domain dataset

[cyrush]

interface strawman:

-
  action: add_pipelines
    p1:
        f1:
           ... my filter, translate dataset, etc
    p2: 
        f1:
           ... my filter, translate dataset, etc
    my_pipeline:
         pipelines: 
              - p1
              - p2
         my_filter:
               type: "combine_pipelines"

Considerations:

• Underlying datasets can be zero copied.
• We will have duplicate domain ids, so domains will need to be renumbered.