Add immunoglobulin nodes to protein tree

[jamesaoverton]

For vertebrates, if ARC classifies a source protein as immunoglobulin, it should go under a special parent node: subject iedb-protein:TAXONID-ab (e.g. iedb-protein:9606-ab), label 'Immunoglobulin chain'. Example (inside the VPN): https://arborist-dev.lji.org/arborist/molecule_tree_old%20molecule_tree/iedb-protein:9606-ab