cross-back-end process graph processing

[JeroenVerstraelen]

Follow up on #95 and #53. Develop a cross-back end process graph splitting implementation that is useful in the current aggregator setup. It should be based on the algorithm developed during the feasibility study.

[soxofaan]

It should be based on the algorithm developed during the feasibility study.

@JeroenVerstraelen can you point me to the details on this feasibility study?

[soxofaan]

There are still some todo's and plenty of idea's for future work, but I decided to merge the current PoC with 9b65520

[soxofaan]

As discussed in #156 the idea is currently to trigger this feature with a job option like

job_options={
    "split_strategy": {
        "crossbackend": {
            "method": "deep"
        },
    }
}