Add @context attribute to CX2 networks

[pkalita-lbl]

Add an @context network attribute in the CX2 conversion process. This will allow IDs to be linked to external resources in NDEx. See https://www.ndexbio.org/viewer/networks/54a9a35b-1e5f-11e8-b939-0ac135e8bacf for an example of a network with a @context attribute. The format appears to be a JSON-encoded string, like so:

"{\"signor\": \"http://signor.uniroma2.it/relation_result.php?id=\", \"BTO\": \"http://identifiers.org/bto/BTO:\", \"uniprot\": \"http://identifiers.org/uniprot/\", \"pubmed\": \"http://identifiers.org/pubmed/\", \"CID\": \"http://identifiers.org/pubchem.compound/\", \"SID\": \"http://identifiers.org/pubchem.substance/\", \"chebi\": \"http://identifiers.org/chebi/CHEBI:\", \"hgnc.symbol\": \"http://identifiers.org/hgnc.symbol/\"}"

JSON-decoded:

{
  signor: 'http://signor.uniroma2.it/relation_result.php?id=',
  BTO: 'http://identifiers.org/bto/BTO:',
  uniprot: 'http://identifiers.org/uniprot/',
  pubmed: 'http://identifiers.org/pubmed/',
  CID: 'http://identifiers.org/pubchem.compound/',
  SID: 'http://identifiers.org/pubchem.substance/',
  chebi: 'http://identifiers.org/chebi/CHEBI:',
  'hgnc.symbol': 'http://identifiers.org/hgnc.symbol/'
}