attr(tweenProp) {
    if (!KEC[tweenProp] && this.valuesEnd[tweenProp]) {
      KEC[tweenProp] = (elem, vS, vE, v) => {
        Object.keys(vE).forEach((oneAttr) => {
          KEC.attributes[oneAttr](elem, oneAttr, vS[oneAttr], vE[oneAttr], v);
        });
      };
    }
  }
onStartAttr.attr

Sets the sub-property update function.
@param {string} tweenProp the property name