Up to now a default easing method 'linear' was used even
if the easing option has been set.
This commit makes sure that the assigned easing option is considered
when animating.w
Currently the `run` method has no return value.
It would be helpful if the method would return a reference
to the animation itselves, so that code like this would
be possible:
````javascript
let foo = new DMX.Animation()
.add(...)
.run(...);
````
