regional-trade example csvw outputs don't include @context

[RickMoynihan]

The .json outputs don't include a CSVW @context and should.

I think the tool creates this now, so suspect they just need to be regenerated.

https://github.com/Swirrl/table2qb/tree/master/examples/regional-trade/csvw

[Robsteranium]

We could do with finishing #123 first.