processors must still have an output file grp

[bertsky]

Despite the changes in the spec long ago, not requiring a processor to have an output_file_grp, core still enforces its defaults, both in the decorators…

https://github.com/OCR-D/core/blob/6ca42f1614fbcaa07801d28d890d104223eb5aa3/ocrd/ocrd/decorators/ocrd_cli_options.py#L25

…and in the Processor constructor…

https://github.com/OCR-D/core/blob/6ca42f1614fbcaa07801d28d890d104223eb5aa3/ocrd/ocrd/processor/base.py#L32

This might go undetected under many circumstances, but the cardinality assertions bring it to the fore:

AssertionError: Expected exactly 0 output file groups, but '['OUTPUT']' has 1