function processEasing(fn) {
    if (typeof fn === 'function') {
      return fn;
    } if (typeof Easing[fn] === 'function') {
      return Easing[fn]; // regular Robert Penner Easing Functions
    }
    return Easing.linear;
  }
Returns a valid `easingFunction`.

@param {KUTE.easingFunction | string} fn function name or constructor name
@returns {KUTE.easingFunction} a valid easing function