[jena] Adding JSON-LD Writer instances

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 ?
> I didn't remove the original definition :
>
> "A typeclass for As supporting transactions."
>
> I just added the @tparam 's .

I know :-) The typo was not caught in an earlier refactoring.

> What I can say in favour of the @tparam is that they provide a structured information, in case tha main paragraph does not provide it.

I was just saying that the tparam was not really useful after the
s/T/A/ fix was done.