Tools for copying files within a workspace

[kba]

@mikegerber https://gitter.im/OCR-D/Lobby?at=5f92cffd6c8d484be2a2448e

Could ocrd-dummy be aliased to ocrd-copy? I have had the need to just copy a file group from time to time, for example to copy a group for subsequent manual corrections. I think this useful operation could have a more deserving name ;)

(or maybe ~ ocrd workspace copy-group?)

I think both requests are easy to implement.

[mikegerber]

Thinking about it, I think it fits as a ocrd workspace command!