Add dutil function to get distinct speaker labels of a single character

[ingoboerner]

I am interested in attaching the unique speaker labels to the character in the RDF rendering by getting distinct-values of the string value in <speaker>. I implemented this in the rdf.xqm which is probably not the best idea because this makes re-using this function much harder.
Consider adding a function to util.xqm:

let $distinct-speaker-labels := $play//tei:sp[@who eq "#" || $character-item?id]/tei:speaker/string()  => for-each(function($label) { replace($label, '\.$', '') }) => distinct-values() 

This results in normalizing "EMILIA" and EMILIA. to a unique value EMILIA, but includes "ODOARDO" and "ODOARDO GALOTTI" as distinct labels.

Consider setting up a range index on the sp[@who] attribute value to speed up the lookup of @who eq "#" || $character-item?id.

[cmil]

The suggested implementation would ignore speeches with multiple speakers. Depending on what you are trying to do with that feature this may or may not affect its usefulness.

[cmil]

Also interesting as kind of an alternative approach, in dracor-org/dracor-schema#81 (comment) @lb42 shared an article describing his practice of adding speaker labels to a <listPerson>. Of course that would require additional work on the TEI documents but with the possible benefit of higher precision.