Enable cross fields search for keyword fields

[ntarocco]

Thanks to the search default operator AND, cross fields settings and search analyzers, it is now possible to search for text in contained in different fields. Example:
https://catalogue.library.cern/search?q=quantum%20field%20theory%20schwartz
See more info here.

However, this does not work:
https://catalogue.library.cern/search?q=BOOKSHOP%20feynman

This is because BOOKSHOP is stored in a keyword field in the OS mappings and analyzers cannot be applied. It would be nice to find a solution to this.