ddl.dic: include `DICTIONARY_AUTHOR` in the `DICTIONARY_VALID` loop

[vaitkus]

Having at least one explicit dictionary author is useful for data provenance purposes. Including the category in the DICTIONARY_VALID loop would allow the validator to automatically report the lack of the corresponding data items. Should this loop be made Recommended or Mandatory?

[jamesrhester]

I think Recommended, to allow maximum policy flexibility for DDL users. Some groups may be philosophically opposed to assigning credit via authorship, although I suppose the group name might then be a reasonable substitute. Within COMCIFS we can adopt a stricter requirement that all dictionaries have at least one author.