Radical

Implement JSON-LD writing for Jena :
    a no-brainer

￼Added some documentation for Generic Parameters, with @tparam :
    it is actually pasted in 3 files; maybe do that only in RDFStore.scala ?
> As seen in class RDFLanguages,
>
> Jena offers only plain vanilla JSON-LD , and rdf-json:

Just provide instances for JsonLd. I don't think anything else is really needed.