Add JSON-LD Context Document for Data Package

[nichtich]

This adds a JSON-LD context for data-package.json with unit test as started to discuss on #869.

[dafeder]

This seems like a good first step but I would think it's of limited usefulness without a namespace for data package itself so that the properties and classes that do not come from other vocabularies can be referenced elsewhere? But maybe outside the scope of this issue?

[nichtich]

The JSON-LD Context does not cover all elements because not all have been mapped to RDF. Limitations are mentioned in README.md. I'd prefer to start with a 80% solution than aiming for 100% first.

[roll]

@nichtich
Great! But as a user, how can I use it? What is the use case and the way to discover it cc @peterdesmet

[nichtich]

The JSON-LD context is used to generate RDF from a Data Package file. Authors of package files should include a reference to it via key @context. This first requires an official location where to host the context file. People using RDF will know how to make use of it but I can also write a short tutorial if needed to show how an example Data Package is represented in RDF. The general use case is data integration: having multiple Data Packages in RDF merged into one graph it's possible to query e.g. all packages by same publisher.

[dafeder]

Just some language cleanup suggestions 😃

[roll]

@nichtich
Shall it be mentioned somewhere in the user-facing part of the site?

[nichtich]

The schema should be referenced from https://specs.frictionlessdata.io/data-package/ but this should better be done after publication of the context document at https://specs.frictionlessdata.io/. I guess this requires some addition to build.js. Better do it step by step, starting with this pull request.