deltaPositionsUpdate(positions) {
    const openPositions = [];

    for (const position of positions) {
      if (position.symbol in this.positions && position.isOpen !== true) {
        delete this.positions[position.symbol];
      } else {
        openPositions.push(position);
      }
    }

    for (const position of Bitmex.createPositionsWithOpenStateOnly(openPositions)) {
      this.positions[position.symbol] = position;
    }
  }
Updates delta positions; Websocket just given use one position per callback

@param positions Position in raw json from Bitmex