Add N-Quads output to phyx.js

[gaurav]

This PR replaces the PhyxWrapper.asOWLOntology() with two methods: PhyxWrapper.asJSONLD() for converting a Phyx file into an OWL/JSON-LD file, and PhyxWrapper.asNQuads() for converting a Phyx file into an N-Quads file, which we do by converting the OWL/JSON-LD file into N-Quads using the jsonld NPM package. While the OWL/JSON-LD files are very useful in development, debugging and testing, we anticipate that most users will use PhyxWrapper.asNQuads() to directly produce N-Quads. It also modifies the phyx2owl.js script so that it produces N-Quad files rather than OWL/JSON-LD files.

Note that we use "N-Quads" instead of "N-Triples" because it is possible to set up a subgraph within a Phyx file using JSON-LD's graph containers, in which case the jsonld NPM package we use should emit those graph IRIs in the N-Quads output. However, I don't know why anybody would do this, and in almost all cases the output we produce will be N-Triples rather than N-Quads. I think "N-Quads" is probably still the right term to use, since that format specifically says the graph IRI may be left out if not needed. But I just wanted to be clear about that.

Should be merged after PR #87.

[hlapp]

My main suggestion is to consider renaming asNQuads() to asRDF(), or even toRDF(). It seems the primary purpose of this method is to produce RDF, and the fact that it accomplishes this by producing NQuads is secondary. Also, it seems safe to say that it will never produce NQuads that isn't RDF. Finally, it would keep it nicely consistent with the method name in the lsonld package.

[gaurav]

Sounds good! I've renamed PhyxWrapper.asNQuads() to PhyxWrapper.toRDF() in 84cd86f.

[hlapp]

👍🏻