code: function normalize() {
           var d = this.length();
  
           if (d > 0) {
             return this._set(this.x / d, this.y / d, this.z / d);
           }
  
           return this;
         }
docs: normalize this vector (scale the vector so that its magnitude is 1)
  @name normalize
  @memberOf me.Vector3d
  @function
  @return {me.Vector3d} Reference to this object for method chaining