add getMarkerPos for sliders

[RebeccaHirst]

add method to get the current marker position as rating (rather than accessing _markerPos) - directly matches method for psychopy requested here psychopy/psychopy#3777

[thewhodidthis]

Hi @RebeccaHirst, thanks for looking into this. As can be seen in the demo, that method is automatically added through the _addAttribute() call in the constructor, no need to explicitly create one.