prefer second position of output_file_grp for AlternativeImage, make FILEGRP_IMG fallback

[bertsky]

Fixed by #59 for binarization already, this needs to be done for all processors. Moreover, this must be reflected in the documentation (ocrd-tool.json, README, docstrings) and tests.

[bertsky]

Addition: since core v2.0.2 (process validation of file groups and parameters) this becomes a must, because without proper description in the tool json we now get:

Exception: fileGrp must not contain commas

[bertsky]

Note: this issue has long since become only about documentation and automatic tests. Probably depends on a better spec.

[bertsky]

Since OCR-D/spec#164 this became obsolete.