@otu missing where required in treebase conversion

[jar398]

ot_31 fails the validator. It comes from the treebase converter, so the converter probably needs to change. Also if it fails to validate I don't know how it got into the repo. Spawned from OpenTreeOfLife/oti#34 .

wget --no-check-certificate -O ot_31.json "https://api.opentreeoflife.org/phylesystem/v1/study/ot_31?output_nexml2json=1.2.1"
... [manually snip off the "data" wrapper ...] ...
./scripts/nexson/validate_ot_nexson.py ot_31.json
{
  "errors": {
    "MISSING_MANDATORY_KEY": {
      "data": [
        "@otu"
      ], 
      "refersTo": {
        "@idref": "Tn295377", 
        "@nodeID": "Tn295377", 
        "@top": "trees", 
        "@treeID": "Tr4206", 
        "@treesID": "Tb6619"
      }
    }
  }, 
...