Add mm:Stanza

[andreamust]

As discussed in the creation of the TunesKG, a new class mm:Stanza is required.
Being stanza a general music term, I propose for the term to be added to this ontology module instead of the Tunes Ontology.

Here a simple conceptualization of the Stanza concept class:

mm:Stanza rdfs:subClassOf mm:CompositionPart ;
    core:hasTextFragment mm:TextFragment ;
    mm:isRealisedIn mm:Score ;
    core:isPartOf mm:AbstractScore .

In other terms, a Stanza is a subclass od a mm:CompositionPart, which refers to a specific mm:TextFragment, and defines its own mm:Score.