Fixed a crash in Double Slider when its min/max values are updated

This is meant to fix the crash in DoubleSlider [reported internally](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-1313).

```DoubleSlider.UpdateValueCore``` method was not implemented and the call fall-through to the base class ```ModelBase``` which throws an exception to highlight the problem.