Validator: Add option to warn about unused / non-standard columns

Might be helpful to spot typos in column names. Those aren't easy to catch, as some columns are optional and additional columns are allowed in most files. Could also implement something smarter, e.g. flagging column IDs with a small Levenshtein distance from standard columns.

The issue names no files or tests. Start by locating the validator and the definitions of standard and optional columns, then determine how warnings should coexist with allowed extra columns. Done should mean the validator can report unused or non-standard columns, with any Levenshtein-based suggestions explicitly defined and tested.