Support passing pg abstractions deeper than 1st argument level

[soxofaan]

fixes #868

Add support for process graph abstractions like DataCube deeper than first level arguments

e.g. (dummy example from unit tests):

settings = con.datacube_from_process(
    "produce_settings", 
    size=4,
)
cube = con.datacube_from_process(
    "colorize", 
    context={
        "color": "red",
        "settings": settings,
    }
)