Refactor Scatter/Bubble Series

Creates a PointSeries as a common base class between points and bubbles.
I'm not sure this is the right entry for the change log since these are internal details.  I think we should only list changes that affect the API of the BubbleSeries or ScatterSeries.

So all I think we need to mention is "Deprecated ScatterSeries.pointRadius().  Use radiusFunction instead." or something similar.