Configuration and REST API support for MARC source codes

[osma]

Some library systems are using our REST API and storing concepts (labels and URIs) in MARC records. In MARC, the sources of concepts are identified by source codes - see e.g. the code list for subject heading and term source codes. Skosmos currently doesn't know about these so the API cannot expose the vocabulary codes that should be used in MARC records, which makes integration with MARC based systems more difficult than it needs to be.

We could support MARC source codes this way:

1. In per-vocabulary config, make it possible to specify MARC source codes using a new property skosmos:marcSourceCode. Like this:

:ysa skosmos:marcSourceCode "ysa" .

or when the code varies by language:

:yso skosmos:marcSourceCode "yso/fin"@fi, "yso/swe"@sv .

2. In the REST API method /<vocid>/ that gives basic information about a vocabulary, expose the codes in a new field, which could be called marcSourceCode in JSON and represented as e.g. skosmos:marcSourceCode in JSON-LD. The code returned by the REST API call would vary according to the lang parameter, just like with titles. If the lang parameter matches one of the language-specific codes, that would be returned; if not, the non-language-tagged version would be returned as a fallback. The marcSourceCode field would not be returned for vocabularies where the code(s) have not been configured.