[feature request] Processor interface

[stweil]

While using ocrd-fileformat-transform, I was thinking about some modifications which I'd like to discuss:

• Wouldn't ocrd-transform be sufficient as name instead of the current lengthy one?
• from-to is a strange parameter name. I see no technical need to inherit the specific needs of ocr-fileformat here and would prefer two parameters from and to. Are there use cases where from cannot be omitted? Technically it should be possible to derive it for all entries in the input file group, and typically it will simply be PAGE. So most users will only have to provide a to parameter or – if it defaults to `ALTO – no parameter at all.