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
@returns {KUTE.easingFunction} a valid easing function