Expand not working if iri already found as leaf in nodes cache

[grtjn]

Keep track which nodes have been expanded, by adding this right after first sparql call:

  newIris.forEach(function(iri) {
    nodes[iri].expanded = true;
  });

And then use this to filter which iris need processing:

  if (!nodes[iri] || !nodes[iri].expanded) {