port processor to core v3

[kba]

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With this PR, eynollah supports OCR-D/core#1240. It simplifies it a lot too.

I'll update the ocrd-tool.json with the changed/added flags here as well.

Draft, please don't merge until v3 stable is released

[bertsky]

BTW, I just tested under (METS Server and) OCRD_MAX_PARALLEL_PAGES=2 – it works, but you need lots of GPU memory, otherwise GPU OOM happens. (It does work with CUDA_VISIBLE_DEVICES=, but of course the CPU utilization grows, so that might stall the system.)

I'm not sure if this warrants adding max_workers = 1 to EynollahProcessor ...