Improved the way DoubleSlider/IntegerSlider loads themselves from file

This pull request fixes the way ```DoubleSlider``` and ```IntegerSlider``` load themselves from DYN file. The ```LoadNode``` method was not calling ```base.LoadNode``` but ```SaveNode``` method was made to call ```base.SaveNode```. This difference has now been removed to align ```LoadNode``` more closely to ```SaveNode``` method, reducing the possibility of introducing error when the ```Slider.LoadNode``` methods are duplicating what ```base.LoadNode``` is already doing.

These changes come after investigating the following defect (but does not fix it):

- [MAGN-2156 Not able to load graph which was created few days back.](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2156)