combining json-ld with api endpoint resolving

[GVogeler]

If we want to achieve a full json-ld based API consumption, it would be good to let the *-refs properties point to the explicit endpoint. This could be achieved in JSON-LD by adding a @context locally, which would replace the @base from the general context-file with the current one.
But:

1. following https://www.w3.org/TR/json-ld/#advanced-context-usage each context definition, will override the original, so the new @base would be valid for all following URI expansions
2. It would be necessary to make this @context required.