static fromJSON (json) {
      return new OperatorNode(
        json.op, json.fn, json.args, json.implicit, json.isPercentage)
    }
Instantiate an OperatorNode from its JSON representation
@param {Object} json
    An object structured like
    ```
    {"mathjs": "OperatorNode",
     "op": "+", "fn": "add", "args": [...],
     "implicit": false,
     "isPercentage":false}
    ```
    where mathjs is optional
@returns {OperatorNode}