search: author search fix

• FIX Fixes an issue of returning the wrong results when searching for single values in the author field (e.g. 'author:ellis').

• Refactors slightly the search_unit logic to be cleaner.

• Adds new search units for special handling of author and subject fields.

• Adds missing pairf property in the IdxINDEX model.

Co-authored-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>