function getCurAxisDir(axis) {
      if (Math.abs(axis.x) > 0.00001) {
        return 'x';
      } else if (Math.abs(axis.y) > 0.00001) {
        return 'y';
      } else {
        return 'z';
      }
    }
Get current axis direction.
@param {CSS3D.Vector3} axis
@return {string} current axis direction.