Add a generator to NDEx CX format

CX format is used by cytoscape and ndexbio (https://www.ndexbio.org/). It would be useful to be able to export to CX from OAK, via the obograph model.

I envision this in two parts:

1. ensuring the core logical content of the ontology (minimally: node.{id,lbl} and edge.{sub,pred,obj}) is exported
2. exporting the kgviz stylesheets to the cx style language

the first should be pretty straightforward, at the core cx is very similar to obographs

```json
"nodes": [
{
"@id": 0,
"n": "ACTA1",
"r": "uniprot:P02568"
},
{
"@id": 1,
"n": "CASP3",
"r": "uniprot:A8K5M2"
},
...
{
"edges": [
{
"@id": 378,
"s": 8,
"t": 11,
"i": "interacts with"
},
{
"@id": 379,
"s": 11,
"t": 16,
"i": "interacts with"
},
...
```

At first I looked into doing this via a linkml model of CX. However, the structure of CX is a little unusual - the top level structure is a list, and different object types form elements of the list, with some assumptions about order. And CX is also self-describing in that a data dictionary of attributes is passed, so there is no one fixed schema of keys for the json.

I tried just manually constructing dict objects but my initial attempts produced invalid structures (I was using the ndex ui to test, not sure if there is a more efficient way)

On slack @bgyori suggests using [ndex2](https://ndex2.readthedocs.io/en/latest/ndex2.html), I think this makes the most sense, although I am loathe to add new dependencies, especially where it may introduce conflicts over shared libraries such as networkx

No file, test, or entry point is named. Start by reviewing the OAK obograph model and ndex2 documentation, then define how the generator should produce CX output; done means exporting node and edge logical content and translating kgviz stylesheets into the CX style language.

Written by the indexing model from the issue text.