sbgn-converter: put metadata to the SBGN-ML (using extensions feature).

[IgorRodchenkov]

Add useful metadata to SBGN nodes/edges (glyphs, arcs, ports) either as nested xml elements ('bp:' or better define own schema if needed) inside <extensions/> and <notes/>, or simply using <![CDATA[...]]> and encoded JSON object in it.

Include, e.g.:

• HGNC, ChEBI, UniProt, PubMed IDs;
• BioPAX Class (name);
• synonyms (as <label/> contains display name already);
• organism(s);
• data source(s);
• evidence ?
• generic or not (boolean)

One way to implement this feature in sbgn-converter, in a very flexible/extensible manner, would be using key:value entries from the Paxtools BioPAX object.annotations (bpe.getAnnotations()) map (and the annotations can be created in many different ways, e.g., within cPath2 or Sifgraph methods).