Add foreign key constraints to CSV-W output for local codelists

[robons]

The CSV-W spec allows you to define foriegn key constraints so that the codes in one table are guaranteed to be deined in a separate table. We can use this functionality to ensure that all of the codes used in a CSV are defined in the relevant code list.

For simplicity in the prototype/MVP we're only planning on doing this validation against locally defined code-lists. Once we have done the work necessary to serialise codelists, we can add foreign key constraints to the CSV-W metadata file which will enforce the relationship.

As part of this task you should write a behave test to ensure that the foreign keys are correctly checked when csv-lint runs.