add bf:qualifier for identifiers

[eroux]

Sometimes identifiers need to be qualified, a common distinction is the isbn for the paperback. On this example it uses:

    bf:identifiedBy [ a bf:Isbn ;
            bf:qualifier "pbk." ;
            rdf:value "9781609452339" ],

which corresponds to the MARC. We should probably limit the number of possible values it can have in SHACL but allowing pbk. would cover most of the cases we have in our database

[xristy]

I'll add to templates/core/identifier.shapes.ttl along w/ the bf:identifiedBy shape

[xristy]

referring to examples at 020 - International Standard Book Number (R), and to bibframe-2 definition:

bf:qualifier
  a owl:DatatypeProperty ;
  dcterms:modified "2016-04-21 (New)" ;
  rdfs:comment "Used with Unspecified" ;
  rdfs:label "Qualifier" ;
  rdfs:range rdfs:Literal ;
  skos:definition "Qualifier of information, such as an addition to a title to make it unique or qualifying information associated with an identifier." ;
.

bf:qualifier appears to be essentially a free text field. Do we want to constrain the possible values, for example via bds:SomeShape sh:in ( "pbk." "hbk." "ebk." ) .?

The example:

020 | ##$a0914378260$qpbk.$qv. 1$c$5.00

also suggests an alternative to [#165], where instead of adding :Volume instances in bdg:MWxxx we attach multiple bf:identifiedBy triples directly to bdr:MWxxx like:

bdr:MWxxx a :Instance ;
    bf:identifiedBy
      [ a bf:ISBN ;
        bf:value "978278452845" ;
        bf:qualifier "pbk." ;
        bf:qualifier "v. 1" ; ] ,
      [ a bf:ISBN ;
        bf:value "978278452633" ;
        bf:qualifier "pbk." ;
        bf:qualifier "v. 2" ; ] ;
.

[eroux]

I think constraining the value via shape is good indeed. I think the idea of adding the qualifiers for the volumes is good too, although I still prefer the first solution, it allows for more expressivity in the future, I'm sure we'll find many cases where we'll need it...

[eroux]

There should also be a "set" value for ISBNs that correspond to sets (as opposed to individual volumes)