Add MRO nodes to protein tree

[jamesaoverton]

Building on #16: For vertebrates, if ARC classifies a source protein as MHC, it should go under a special parent node: subject iedb-protein:TAXONID-mhc (e.g. iedb-protein:9913-mhc), label 'Immunoglobulin chain'. It should also include a slice of MRO (MHC Restriction Ontology). Example (inside the VPN): https://arborist-dev.lji.org/arborist/molecule_tree_old%20molecule_tree/iedb-protein:9913-mhc.

• Bos taurus (bovine) protein [iedb-protein:9913]
  • Major Histocompatibility Complex chain [iedb-protein:9913-mhc]
    • MHC-I chain [iedb-protein:9913-mhc-i]
      • MHC-I alpha chain [iedb-protein:9913-mhc-i_alpha]
        • BoLA-2 chain [MRO:0000356]
          • BoLA-2*01801 chain [MRO:0000358]
            • BOLA class I histocompatibility antigen, alpha chain BL3-7 [UP:P13753]

A bunch of these ID should use MRO.