allow re-runs

[bertsky]

Sometimes, processing might fail due to temporary downtimes. Or bugs in tools which get fixed subsequently.

Regardless, it should be easy to re-run the same workflow on a workspace again. To that end, ocrd process offers --overwrite (as does ocrd workflow client process, and ocrd-make always uses it).

But what about badly written workflows or data imported from presentation – is overwrite always the right thing to do?

[bertsky]

Besides incremental processing (for which ocrd process --override does help), we also have the initial, unconditional step of ocrd-import of workspace (which is not possible if the METS already exists).

And we must now consider that residual data lives under either:

• the /data volume of the Controller, under $REMOTEDIR, named KitodoJob${PID}_$(basename ${PROCDIR}):
  the full workspace including images and mets.xml
• the /data volume of the Manager, under $WORKDIR, named ocr-d/$PROCDIR:
  the full workspace including (refcopy of) images and mets.xml
• the /data volume of the Manager and Production, under $PROCDIR, named $PROCDIR:
  currently images and ocr results