Refactor Scatter/Bubble Series

Creates a PointSeries as a common base class between points and bubbles.
The API Docs don't currently include any public members of PointSeries so the ScatterSeries and BubbleSeries documentation pages look a bit sparse!

If we add the /** to the top of this class then it will show up in the API docs but this is still not a great solution since the user has to know that BubbleSeries and ScatterSeries inherit from PointSeries in order to discover what they can do with them.

JsDocs has an [@augments](http://usejsdoc.org/tags-augments.html) tag that could be useful.

Anyway I think this is an issue that is wider than this PR so I'm adding an issue to Jira for documentation of inheritance.