Cross-backend processing: more tuning parameters

[soxofaan]

Spin-off/follow-up issue for #150 ("deep" graph splitting):

the "deep" graph splitting feature turned out to have more aspects and considerations than anticipated. For initial implementation some ad-hoc decisions were taken, but once we have more experience with actual usage and challenges, it will become more clear what can be improved and what finetuning options make sense.

Some initial finetune ideas:

• preferred back-end for main graph
• deny-list for nodes not to split on
• force-list for nodes to split on
• maximum depth to split on